IRC : 21-2000

STANDARD SPECIFICATIONSAND

CODE OF PRACTICEFOR

ROAD BRIDGES

SECTION: III

CEMENT CONCRETE(PLAIN AND REINFORCED)

(Third Revision)

THE INDIAN ROADS CONGRESS2000

IRC: 21-2000

STANDARD SPECIFICATIONSAND

CODE OF PRACTICEFOR

ROAD BRIDGES

SECTION: III

CEMENT CONCRETE(PLAIN AND REINFORCED)

(Third RevLcion)

PublishedbyTHE INDiAN ROADS CONGRESSJamnagarHouse,ShahjahanRoad,

New Deihi-liflOll2000

Price Rs./60/-(Pluspacking & postage)

IRC: 21-2000

First published

ReprintedFirst RevisionReprinted

Reprinted

ReprintedReprinted

SecondRevisionReprinted

Reprinted

ReprintedReprinted

Third Revision

October, 1966

January, 1969March, 1972July, 1974

April, 1977

June,1980March, 1983

March, 1987August, 1988September,1994

November,1996

December,1997October,2000

(IncorporatesAmendmentNo.1-Nov.1971)

(IncorporatesAmendmentsNo,2-Dec,1972,No,3-Nov,1973andNo.4-April, 1974)(IncorporatesAmendmentNo.5-April, 1976)

(IncorporatesAmendmentNo.6-May, 1977)(IncorporatesAmendmentsNo.7-September1981 andNo.8-December1982,modificationof ClauseNo. 303.5,changesin Notationsand unitsas per IRC:71)

(IncorporatesAmendmentto Clause304,7.1,1ErrataandAmendmentNos, I to SecondRevision)

(Rights of Publication and of Translation are reserved~?

Printed at Dee Kay Printers,New Delhi-I 100151000 Copies - October, 2000

IRC: 21-2000

CEMENT CONCRETE (PLAIN AND REINFORCED)

CONTENTS

PageNo.Compositionof Bridge Specifications & StandardsCommittee (i) to (ii)

Notations (iii) to (v)

Clause No.

Introduction 1

300 Scope 2

301 Terminology 3

302 Materials 4

303 Basic PermissibleStresses 18

304 GeneralDesign Requirements 20

305 Beams and Slabs 43

306 Columns and Compression Members 58

307 Footing or Bases 65

308 Balanced Cantilever andContinuousConstruction 69

309 Box Girders 70

310 Workmanship 71

79Appendix I Design SurfaceCrack Width

IRC 21-2000

MEMBERS OF THE BRIDGE SPE~IFICATIONSAND STANDARDS COMMITTEE

(As on 7,12.99)

I. Prafulla Kumar*(Convenor)

2. TheChiefEngineer(B)S&R (Member-Secretary)

3. S.S. Chakraborty

4. Dr. MG. Tamhankar

5.

6.

Ninan Koshi

AG. Borkar

7. N.K. Sinha

8. M.V.B. Rao

9. CR.Alimchandani

10. Dr. S.K. Thakkar

IL M.K.Bhagwagar

12. P.D. Wani

13. S.A. Reddi

14. Vijay Kumar

15. C.V.Kand16. M.K. Mukherjee17. MaheshTandon

18. Dr. TN. Subba Rao

19. AK. Hant

20. AD. Narain

21. V.R.Jayadas

22. P.C. Bhasin

DG(RD) & AddI. Secretary, Ministry of SurfaceTransport(RoadsWing), TransportBhawan,NewDelhi-I 10001(V. Velayutham), Ministry of Surface Transport(Roads Wingh), Transport Rhawan, NewDelhi-i 10001Managing Director, Consulting Engg. Services(1) Ltd., 57,Nehru Place,New Delhi-I 10019Emeritus Scientist, StructuralEngg. Res. Centre, 399, PocketE, Mayur Vihar Phase 11, Delhi-I 10091DG(RD) & Add!. Secretary(ReId.), 56, Nalanda Apartments,Vikaspuri, New DelhiTechnical Adviser to MetropolitanCommr., A-I, Susriehi PlotNo. 22, Arun Kumar Vaidya Nagar, Bandra Reclamation,Mumbai-400050Chief Engieer (PLC), Ministry of Surface Transport (RoadsWing), TransportBhawan, New Delhi- 110001Head, Bridge Division, Central Road Researchinstitute, P.O.CRRJ,New Delhi-I 10020Chairman & Managing Director (STUP Consultants Ltd.,1004-5, Raheja Chambers, 213, Nariman Point, Mumbai -400021Professor, Department of Earthquake Engg., University ofRoorkee,Roorkee-247667Consulting Engineer, Engg. Consultants (P) Ltd., F-14/IS,Connaught Place,NewDelhi-I 10001Member, MaharashtraPublic ServiceCommission, Bank ofIndia Building, Mumbai-400025Dy. Managing Director, Gammon India Ltd., Gammon House,Prabhadevi,Mumbai-400025Managing Director, UP State Bridge CorporationLtd., SewBhavan, 16, MadanMohan Malviya Marg, Lucknow-226001Consultant,E-2/136,Mahavir Nagar, ~hopai-46201640/I 82, C.R. Park, NewDelhi-I 10019Managing Director, Tandon Consultants(P) Ltd., 17, LinkRoad, Jangpura, Extn., New DelhiChairman, ConstrumaConsultancy(P) Ltd., 2nd Floor, PinkyPlaza,Mumbai-400052Executive Director (B&S), Research Designs & StandardsOrganisation,Lucknow-226001Director General (Road Development)& Addl. Secy.,MOST(Retd.), B-186, Sector 26, NOIDAChief Engineer,Dy. Director General (B), DGBR, SeemaSadakShavan,Delhi Cantt.,Naraina,NewDelhi-66324,MandakiniEnclave, Alkananda,New Delhi-I 10019

*ADG(B) being not in position. The meeting waspresided by Shn Prafulla Kumar, Director

General (Road Development)and AddI. Secretary,Ministry of Surface Transport.

(i)

IRC: 21-200023. P.K.Sarmah

24. S.R.Tambe

25. The Secretaryto theGovt. of Gujarat

26. The ChiefEngineer(R&B)

27. The Chief Engineer(NH)

28. The Engineer-in-Chief

29. The Director General(Works)

30. The ChiefEngineer3!. The ChiefEngineer(R)

S&R32. The Director

33. The Director & Head

34. The Chief Engineer (NH)35. The Chief Engineer (NH)36. The Chief Engineer (NH)

37. PresidentIndian Roads Congress

38. Director General(Road Development)

39. Secretary,Indian RoadsCongress

I. NV. Merani

2. Dr.G.P.Saha

3. Shitala Sharan

Chief Engineer, PWD (Roads) Assam, P.O. Chandman,Guwahati-781003Secretary,MaharashtraPWD(RaId.),72, PranitI. PalkarMarg,Opp.PodarHospital, Work,Mumbai-400025(H.P. Jamdar) R.&B Department,Block No. 14,NewSachivalaya,2nd Floor, Gandhinagar-382010(D. SreeRams Murthy), National Highways, Irrum Manril,Hydetabad-500482(D. Guha), Public Works Department, Writera’ Building, BlockC, Calcutta-700001(R.R. Sheoran), Haryana P.W.D., B&R, Sector-19 B,Chandigarh-160019CentralPublic WorksDepartment, NirmanShawan,NewDelhi

Ministry ofSurfaceTransport(Roads Wing), NewDelhi(CC. Bhattacharya), Ministry of SurfaceTransport(Roads Wing), TransportShawan,NewDelhiHighways ResearchStation, 76 Sarthat Pate! Road,Chennai-600025.Bureau of Indian Standards, Manak Bhawan, 9, BahadurshahZafar Marg, New Delhi-i 10002M.P. Public Works Department,Bhopal-461004LIP. P,W.D., LucknowPunjabPWD,B&R Branch,Patiala

Ex-Officlo MembersK.8. RajoriaEngineer-in-Chief,Delhi, PWD,NewDelhiPrafulla Kumar,DG(RD) & Addi. Secretaryto the Govt.of India,Ministry of SurfaceTransport(RoadsWing),TransportBhawan,NewDelhi-i 10001S.C. Shaima,ChiefEngineerMinistry ofSurfaceTransport(RoadsWing)TransportBhawan,NewDelhi-I 10001

Corresponding MembersPrincipal Secretary(Retd.),A-4711344,AdarshNagar,Worli, Mumbai.400083ChiefEngineer,HindustanConstructionCo. Ltd.,Hincon House,Lal BabadurShastri Marg,Vikhroli (W), Mumbai-400083Advisor Consultant, ConsultingEngg. Services(I) P~rt.Ltd.,57, NehruPlace,NewDethi-l 10019

(ii)

IRC 21-2000

NOTATIONS

Es = modulusof Elasticity of steelEc = modulusof Elasticity of concretelo = Basic anchoragelengthn = Factorfor basic anchorage length4) = diameterof reinforcingbars

= Design anchoragelength= factor for reduction in anchorage length

= Factor for reduction inanchoragelengthfor excessreinforcementAs = areaof bars actuallyprovided

= areaof bars required at fullpermissible stressV = Shift of curtailment line

= Shift of curtailmentline at endsupportsla = Lap lengthR = Supportreactionh Depth of Sectiona = axial spacingof the bar for calculating the laplength

(ci. 304.6.6.3)or the distanceof centre of gravity of the concentratedloadfrom the nearersupport(ci. 305.16.2)

b = edge distaneof barfor calculationof lap length (cl. 304.6.6.3)or thebreadthof the rectangularbeamor slab or thebreadthofthe rib in caseof flanged beam(ci. 304.7.1.1.1)

V = the design shear across the sectiond = effectivedepthof the section,beam or slab13 = The angle between the top andbottom edgesof the beam or

slabof varying depth at thesectionconsidered for shear(ci. 304.7.1.1.2)or coefficient for reduction in maximum permissible stresses inlong column(ci. 306.4.3)or Ratio of the long side to the short side of the footing (cI.307.2.4)

P = axial compressiveforce in Newtons,Ag = gross areaof the concrete section inmm2t = Design ShearStressat anycrosssection

= Maximum permissible Shear Stress= totalcross-sectional areaof stirrup legs or bent up barswithin

a distances

(iii)

IRC: 21-2000

s = spacingof the stirrups or bent-up bars along thelength of themember

= permissibletensile stressin shear reinforcementa = angle between the inclinedstirrup or bent upbarand theaxis

of the member,not less than 450 or (cl.307.l.4) or constant forcalculation of effective width (cl.305.16.2) or modular ratio(ci. 306.5.2)

Ve equivalentshearT = torsional momentM = bending moment at the cross-section

= equivalent BendingMomentdue to torsionD = the overall depthof the beam

= cross-sectional areaof bar forming the closed hoop= centreto centre distance between corner bars in the directionof

the widthd, = centre tocet~tredistance between cornerbarsin the directionof

the depth= equivalent shearstress= shear strengthof the concrete

1~ = effective span (ci. 305.4) or the unsupportedlength of thecompression flange (cl.305.9), orthedistancebetween pointsofzero moments(Cl. 305.15.2)

/ = distance between centreofsupports Clause305.15.2or effectivelength of column (cl. 306.4.3)

1~ = clear spand = effectivedepth of beam orslabb = effective width for compressionflange

= thicknessof the web for beams(for T-beam and slab) orthicknessof the webs+ width of the slab betweenwebs (forhollow box sections)

b1 = the effective widthof slab on which theload acts= thebreadthof concentration areaof the load for calculationof

effective widthc.ro = the permissible stress in direct compressionfor concrete

= the permissiblestressin direct compression for the longitudinalsteel

A = the cross-sectional areaof concreteexclusiveof any finishingmaterial applied after the castingof the column

(iv)

IRC: 21-2000

A, = the cross-sectionalareaof the longitudinal steel in columns orareaof the tensionreinforcement

Ao = the cross-sectional area of concrete in the column core,excludingthe areaof longitudinal steel

= the equivalentarea of helical reinforcement= Tension in helical reinforcement

r = the least radiusof gyrationN = the permissibleaxial load on the columnW = the section modulusup, cal = the calculated direct compressivestresso~,cal = the calculated compressive bendingstress

= the permissibleflexural compressivestressAL = dispersed concentric area which isgeometricallysimilar to the

loaded areaA2 = loadedareac = is the minimum cover to the tension steel

= is the perpendiculardistance from the point considered to thesurfaceof the nearestlongitudinal bar

c = is the averagestrain at the level where crackingis beingconsideredcalculatedallowing for the stiffening effect of theconcrete inthe tension zone

b = is the widthof the sectionat the centroidof the tensionsteela = is the distance fromthe compression face to thepointat which

the crack widthis being calculatedC = is the averagestrain at the level where cracking is being

considered, calculated ignoringthe stiffening effect of theconcrete in the tensionzone

x = is the depthof the neutralaxis found from the analysis todeterminee,

= characteristicsstrengthof untentionedsteel in N/mm2

(v)

IRC: 21-2000

CEMENT CONCRETE (PLAIN AND REINFORCED)

INTRODUCTION

The StandardSpecificationsand CodeofPractice for RoadBridges, Section III - Cement Concrete (Plain and Reinforced)wasfirst published in October, 1966.To cater for thetechnologicaldevelopmentswhich were taking place in course of time, theCode was exaniineçl by the Technical Committees of the IRCand revised in 1972 and in 1987 in the light of theirrecommendations.

In the light of further developmentsin the field of plainand reinforced concrete, the provisions of the Code werereviewed by the Committee for Reinforced, PrestressedandCompositeConcrete (13-6)consistingofthe following personnel:

CORRESPONDING MEMBERP.S.Tyagi

NinanKoshiA.N. Dhodapkar

N.K. SinhaS.G. JoglekarDr. T.N. SubbaRanM.K. MukhexjeeDr. M.G. TainhankarShitala SharanMaheshTandonS.A. ReddiAshok K. Basa

ConvenorMember-Secretary

MEMBERSJoseKurienP.Y. ManjureA.S.PrasadaRaoitA. ChaudharyG.R. HaridasD.D. SharmaDirector& Head,Civil Engg.,Bureauof Indian Standard,(Vinod Kumar)

T. ViswanathanEX-OFFICIO MEMBERS

President, IRC DG(RD)(K.B. Rajoria) (PrafullaKumar)

Secretary, IRC(S.C. Sharma)

1

IRC: 21-2000

The amendments as finalised by this Committee wereconsideredand approved by the Bridge SpecificationsandStandardsCommitteein theirmeetingheldat NewDelhi on 7thDecember,1999andlater approved by theExecutiveCommitteein their meeting held atNew Delhi on 14th December,1999.The, draft amendments werediscussedand approved by theCouncil of the Indian Roads Congress at the157th CouncilMeeting held at Maduraion 4th Jahuary,2000.

It wasalso decided that the document would bepublishedas a fully revised Codeafterincorporatingall the amendments.

The objectof issuingthe Standard Specificationsand CodeofPractice for RoadBridges Section111-CementConcrete (Plainand Reinforced) is to establish a commonprocedurefor thedesignand constructionofroad bridges in India. The publicationis meant toserveas a guideto both the design engineers and theconstruction engineers but compliance with the provisionsthereindoesnot relievethemin any wayofthe responsibilityforthe stability,soundnessand safetyofthe structuresdesignedanderected bythem.

The designand constructionof road bridges requireanextensiveand thorough knowledgeof scienceand techniqueinvolved and should be entrusted only to specially qualifiedengineerswith adequate experienceof bridge engineering,capable,of ensuringcareful executionof work.

300. SCOPE

This Code deals with thestructuraluseof plain cementconcreteand reinforcedcementconcrete in roadbridges.

2

IRC: 21-2000

301. TERMINOLOGY

For the purposeof this Code, the following definitionsshall apply:

301.1. Concrete

A mixture of cement, water,fine and coarse aggregatesand anyadmixture (that may be permittedby the competentauthority) preparedand placed in accordance withthis Code.

301.1.1. Reinforced concrete: Concrete containingsteel reinforcement (non-prestressed)conforming to Clause302.5 of this Code and of no less than theminimum amountrequired by this Code andis a composite materialin which bothmaterials actin co-operationto resist the stress resultants.

(Concrete containing prestressed steel reinforcementforintroducing precompressionoftensile flangeofelementis termedPrestressedConcretewhich is outside the purviewof this Code).

301.1.2. Plain concrete: Concretecontainingno steelreinforcement orless amountof reinforcement than specifiedforreinforced concretein this Code, the co-operationof such steelbeing ignoredin resisting stress resultants.

(Plain concrete cast in such massive dimensions as torequirespecific measuresto be takento caterfor generationofheatandattendant volume change to minimise crackingis termedMpssCo9crete whichis outside the purviewof this Code).

301.2. Core of Helically Reinforced Column Theportionof the concrete enclosed within the outer surface formedby the helicalreinforcement.

301.3. Curing Concrete : Maintaining moisturecondition to promote continued hydration of cement in theconcrete.

3

1RC: 21-2000

301.4. EffectIve Depth of a Beam : The distancebetween the centroidof the areaof tension reinforcement andthe fibre at which the compressivestressis maximum.

301.5. Mortar : A mixture of cement, fine aggregateand water and any admixture that may be permitted by thecompetentauthority.

301.6. Cover : Thethicknessfrom theoutersurfaceofthe concrete to the nearest surfaceof the reinforcement.

302.1. Cement302. MATERIALS

Any of the following typesof cementmaybe usedwithprior approvalof competentauthority.

Table 1

Type Conformlag to

{i)(ii)

Ordinary PortlandCement33 GradeOrdinaryPortlandCement43 Grade

IS: 269IS: 8112

(iii) OrdinaryPortlandCement53 Grade IS:12269(iv) RapidHardeningPortlandCement IS: 8041(v) SulphateResistantPortlandCement IS: 12330(vi) PortlandPozzolanaCement IS: 1489-Part-I(vii) PortlandBlast FurnaceSlag Cement IS: 455(viii) Low Heat Portland Cement IS: 12600

Notes : (1) be permittedonly inUse of Portlandpozzolanacementmayplain concretemembers.

(2) Under severe conditionof sulphate content insubsoil water,special literatureon precautionsto betakenwith regardto theuse of specialtypes of cementwith low C3A contentmay bereferr&~’to.Durability criteria like minimum cementcontentand water cement ratio, etc. should also be given dueconsideration.

4

IRC: 21-2000

302.2. Admlxtures

To improveworkability ofconcrete,admixtures,conformingto IS:9103 may be used.

302.3. Aggregates

302.3.1. General : All coarse and fine aggregates shallconform to IS:383 and shall be tested to conform to IS:2386PartsI to VIII.

302.3.2. Coarseaggregate

302.3.2.1.Coarse aggregates shall consistof clean, hard,strong, dense,non-porousand durablepiecesof crushed stone,crushedgravel,naturalgravel or a suitable combinationthereofor other approved inertmaterial.

302.3.2.2. The maximum sizeofthe coarse aggregate maybe as large as possible within the limitsspecified,but in no casegreater than one quarterofthe minimum thicknessofmemberor10 mm less than theminimum lateral clear distancebetweenindividual reinforcementsor 10 mmlessthan the minimumclearcover to any reinforcement.

302.3.2.3. The preferred nominal sizeof aggregateis 20mm for reinforced concreteand prestressedconcrete.However,larger sizes upto40 mm may bepermittedin special cases whenthereis no restriction to flowof concrete in a section.

For plain concrete, preferred nominal sizesmay bebetween20 mm and 40 mm. However,larger si~esmay bepermitted only in special cases, subject to supplemen~taIspecificationsandprecautions.

302.3.3. Fineaggregates: Fine aggregates shall consistof hard, strong,durable clean particlesof natural sand,crushed

5

IRC: 2 1-2000

stone or gravel or suitable combination of natural sand andcrushedstoneor gravel.They shall not containdust, lumps, softor flaky particles, micaand other deleterious materialsin suchquantitiesaswould reduce the strengthor durabilityof concreteor attack the reinforcement.

302.3.4. Grading of aggregatesshall be such as toproducea dense concreteof thespecifiedstrength, which can beworked readilyinto position withoutsegregationand without theuseof excessivewater content.

302.4. Water

Waterusedfor mixing and curing shall beclean and freefrom injurious amountsof oils, acids, alkalis, salts, sugar,organicmaterialsor othersubstancesthat may be deleterioustoconcreteor steel.

302.4.1. In case of doubt regardingdevelopmentofstrength, thesuitability of water for making concrete shall beascertainedby the compressive strength andinitial setting timetests specified in 302.4.1.2.and 302.4.1.3.

302.4.1.1. The sampleof water taken for testingshallrepresent the waterproposedto be usedfor concreting,dueaccount being paid to seasonal variation. The sample shall notreceiveany treatment before testingotherthan thatenvisagedinthe regularsupply of waterproposedfor use inconcrete.Thesample shall be storedin a cleancontainer previously rinsed outwith similar water.

302.4.1.2, Average 28 days compressivestrength of atleast three 150 mm concrete cubes prepared with waterproposedto be usedshall not beless than 90 per centof theaverage of strengthof three similar concretecubes preparedwith distilled water. Thecubes shall be prepared, curedand

6

IRC: 2 1-2000

testedin accordance with the requirementsof IS:516.

302.4.1.3. The initial setting time of testblock madewiththe appropriatecementand the waterproposedto be usedshallnot be less than 30 minutes and shall not be more than30minutes from the initial setting time of control test blockpreparedwith the same cement and distilled water. The testblocks shall be preparedand tested in accordance with therequirementsof IS:403l (Part 5).

302.4.2. The pH valueof water shall not belessthan 6.Potable water is generally consideredsatisfactory for mixingconcrete.As a guide the following concentrationsrepresentthemaximum permissible values

(a) To neutralise100ml sampleof water,using phenolphthaleinasan indicator, it should not require more than5 ml of 0.02normal NaOH. Thedetails of test are given in8.1 of IS:3025(Part 22).

(b) To neturalise100 ml sampleof water,usingmixed indicator, itshould not require more than 25 ml of 0M2 normal H2S04. Thedetailsof test shall beas given in Clause8 ofIS:3025(Part23).

(c) Permissible limits forsolids shall be as given in Table 2.

Table 2. Permissible Limit for Solids

Testedas per Permissible limit max.Organic IS:3025 (Pt. 18) 200 mg/lit.Inorganic IS:3025 (Pt. 18) 3000 mg/lit.Sulphates (asSO3) IS:3025 (Pt. 28) 400 mg/lit.Cholorides (as

~Cl) IS:30254~Pt.32) 2000 mg/lit.

for concrete worknotcontaining embeddedsteel and500 mg/lit, forprestressedlreinforcedconcretework

Suspendedmatter IS:3025 (Pt. 17) 2000 mg/lit.

7

IRC 21-2000

302.4.3. Mixing or curing of concretewith seawater isnot permittedbecauseof presenceofharmful saltsin sea water.

302.4.4. Water found satisfactory for mixing is alsosuitable for curing concrete.However, waterused for curingshould not produce any objectionable stainor unsightlydepositon the concretesurface. The presenceof tannic acid or ironcompounds is objectionable.

302.5. Reinforcement

Reinforcement shall consistof the following gradesofreinforcing bars, designated bytheir characteristic strength,where characteristicstrengthf~shall be taken as that specifiedingoverning IS Specificationslisted in Table 3 asthe minimumvalue of 0.2 per cent proof stress oryield stress.

Table 3

Gradedesignation

Bar type conformingto governing ISspecifications

Characteristicstrengthf MPa

Elastic modulusGPa

Fe 240 JS:432Part1Mild steel

240 200

Fe 415 IS:1786 Deformedbar

415 200

Fe 500 IS:l786 Deformedbar

500 200

Note: Wire fabrics conforming to IS: 1566 and TMT bars conforming toIS:1786 can also be used.

8

IRC : 21-2000

302.6. Concrete

302.6.1. Concrete grade : The concrete shall be ingradesdesignated in Table4, where the characteristic strengthisdefined as the strengthof material below which not more than5 per centof test results are expected to fall

Table 4

Grade designation

M 15M 20M 25M 30M 35M 40M 45M 50M 55

Specifiedcharacteristiccompressivestrengthof

150mm cubesat 28 days(MPa)

152025303540455055

302.6.2.Minimum concretegrade,minimum cementcontentand maximumwatercementratio for structural~membersunderdifferent conditionsof exposureare givenin Table 5.

302.6.3.The cement content shall not exceed540 kg/rn3 ofconcrete.

302.6.4.Total water soluble sulphate(SO3) content of the

concrete mixexpressedas (SO3)shall not exceed4 percent bymassof cement used in the mix.

302.6.5. Total chloride content in concrete expressed aschloride-ion shall not exceed the following values by massof

9

IRC

:21-2000z

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10

IRC : 2 1-2000

cementused

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PSCR.C.C. (in severe condition of exposure)R.C.C. (in moderate conditionof exposure)andP.C.C.

Per cent

~0,l00.200.30

302.7. Requirement for Design Mixes

302.7.1. Target mean strength : The target meanstrength of specimenshall exceedthe ~specified characteristicstrengthby at least thecurrentmargin

(i) The currentmargin for a concrete mixshall bedeterminedandshall be taken as 1.64 times the standarddeviationof sampletest resultstaken from at least40 separatebatchesof concreteof nominally similar proportionsproducedat site by the sameplant undersimilar supervision,overaperiod exceeding5 days,but not exceeding6 months.

(ii) Where thereare insufficient data to satisfy the above, thecurrent marginfor the initial mix designshall betakenasgivenin Table 6.

Table 6

Concrete Grade Current Margin(MPa)

Target Mean Strength(MPa)

M 15 10 25M 20 10 30M 25 11 36M30 12 42M 35 12 47M40 12 52M45 13 58M 50 13 63M 55 14 69M 60 14 74

11

IRC: 21-2000

This initial currentmargin, given in Table 6, shall beusedonly until sufficient data are available to determine the currentmargin as per sub-clause(i) above.

302.7.2. SuitabilIty of proposed mix proportionsFollowing information shall be furnished for deciding suitabilityof proposed mix proportions

(a) Natureand sourceof each material

(b) the quantitites of each materialper cubic metre of fullycompacted concrete

(c) either of the following(i) appropriate existing dataas evidence of satisfactory

previousperformancefor targetmeanstrength, currentmargin,workabilityandwater/cementratio and anyotheradditional requirement(s)as specified.

(ii) full details of tests on trial mixes carried out inaccordancewith sub-clause302.7.3 or

(iii) for ordinary structural concrete, a statement that, forinitial production,the appropriatemix proportionsgivenin Table 7 will be used.

Table 7. Proportionsfor Nominal Mix Concrete

ConcreteGrade

Total quantity of dryaggregate by mass per50 kg of cement to betaken as the sum ofindividual masses offine and coarse

Proportionoffine aggregateto coarseaggregate

(by mass)

Maximum quantityofwater per 50 kgof cement(Litres)

aggregate (Kg) P.C.C. R.C.C.

MiS 350 Generallysubjecttolimit 1:1.5and lowerlimit 1:2.5

1:2,upper

25 -

M20 250 25 22

12

IRC : 21-2000

302.7.3. Trial mixes : Trial mixes shall bepreparedusing samplesof approved materials forall gradesof concrete.

Sampling and testing procedures shall bein accordancewith Clauses302.10and 302.11.

The concretingplant andmeansoftransportationemployedto make thetrial mixes and to transport them to representativedistances shall be similar tothe corresponding plant andtransportto be usedin the works. A clean drymixer shall beused and the first batches shall be discarded.Testcubesshall betakenfor trial mixes asfollows. For eachmix, set of six cubesshall be made from eachof threeconsecutivebatches.Threefrom eachset of six shall be tested at an ageof 28 days andthreeat an earlier age approved by the engineer-in-charge. Thecubes shall be made, cured,stored, transportedand testedincompressionin accordance with thespecification.

The average strengthof the nine cubes at28 daysshallexceed thespecified characteristic strength by thecurrentmargin minus 3.5 MPa.

302.7.4. Additional trial mixes : Additional trial mixesandtests,shall becarriedout duringproduction beforesubstantialchanges are madein the materialor in the proportionsof thematerials to be used,except when adjustments to themixproportions arecarried out in accordance with302.9.2.1.

302.8.. RequIrement for Nominal Mix Concrete

Unlessotherwisespecified,the nominal mixconcreteshallbe as detailedin Table 7.

302.9. Productionof Concrete

302.9.1. Batching and mixing : The quantities ofcement,fine aggregate and the varioussizesof coarse aggregate

13

IRC : 21-2000

shall bemeasuredby weight,unlessotherwise authorised by theengineer-in-charge.

A separate weighing machine shall be provided forweighing thecement.Different types of cement shall notbemixed.

Thequantityofwater shall be measured. Any admixturetobe added shall be measuredand, if solid, shall be measuredbyweight.

The batch weightof aggregateshall be adjustedto allowfor moisturecontenttypical of the aggregatebeingused.

All measuring equipment shall be maintainedin a cleanand serviceablecondition. Its accuracy shallbe checked over therangein usewhenset upat each site, and maintained thereafter.

The accuracyof equipmentshall fall within the followinglimits

Measurementof cement ± 3 per cent of the quantityof cementin eachbatch

Measurement of water ± 3 percentof the quantityof water ineach batch

Measurementof aggregate ± 3 per centof the quantityof aggregatein eachbatch

Measurementof admixture ± 5 percentofthequantityof admixturein eachbatch

The concretemixer shall comply with therequirementofBIS whereapplicable.The mixing time shall be notless thanthat recommended by the manufacturer, subject to the approvalof the trial mixes by theengineer-in-charge.

Concretemixers that have been outof usefor more than30 minutes shall be thoroughly cleanedbefore any fresh

14

IRC : 21-2000

concreteis mixed. Unless otherwise agreed thefirst batch ofconcrete through the mixer shall thencontainonly two-thirdsofthe normal quantity of coarseaggregate.Mixing plant shall bethoroughly cleanedbeforechangingfrom one type ofcementtoanother.

302,9.2. Control of strength of designed mixes

302.9.2.1.Adjustment to mix proportions : Adjustmentto mix proportions may be madein order to minimise thevariability of strengthsubjectto approvalof engineer-in-chargeand to maintain the target mean strength subject to approvalofengineer-in-charge.Suchadjustments shall not be taken to implyany changein the current margin.

302.9.2.2.Changeof current margin Whenrequiredbythe engineer-in-charge,the current margin shall berecalculatedin accordance with Clause302.7.1.The recalculatedvalueshallbe adopted as directed by theengineer-in-charge,and it shallbecome the current marginof concrete produced subsequently.

302.10. Samplingand Testing

302.10.1. General: Samplesfrom freshconcrete shall betaken asperiS: 1199and cubesshall be made, cured and testedat 28 days in accordance withIS:516.

302.10.2. Sampling procedure A random samplingprocedure shall be adoptedto ensure thateachconcretebatchshall have a reasonable chanceof being tested, thatis, thesamplingshould be spreadover the entire periodof concretingand coverall mixing units.Thepoint andtime ofsampling shallbe at delivery into theconstruction,unlessotherwise agreed to.

302.10.3.Frequency:The minimumfrequencyofsamplingof concreteof each grade shall bein accordance with Table8.

15

!RC: 2 1-2000

Table 8Quantity of Concrete in Work (m3) Number of Samples

1-5 1

6-15 216-30 3

31-50 451 and above 4 plus one additional

sample for each additionai50 m3 or part thereof

At leastonesample shall be taken from each shiftofwork.

302.10.4. Test specimen and sample strength : Threetest specimens shall be madefrom eachsamplefor testing at28days. Additional cubes may be requiredfor various purposessuch as to determine the strengthof concrete at7 days for anyother purpose.

Theteststrengthofthe sample shall be theaverageofthestrengthofthreespecimens. The individual variationshouldnotbe more than± 15 per centof the average.

302.11. AcceptanceCriteria

302.11.1. Compressive strength : When both thefollowing conditions are met, the concrete complies withthespecifiedcompressivestrength:

(a) The mean strengthdetermined from any group of fourconsecutivesamplesshould exceed the specifiedcharacteristiccompressive strength by 3 MPa.

(b) Strengthofany sampleis not lessthan thespecifiedcharacteristiccompressivestrengthminus3 MPa.

302.11.2. Workability : The concrete mix proportionschosen shouldbe such that the concreteis of adequate

16

IRC : 21-2000

workability for placing conditionof the concrete to ensurepropercompaction. Suggestedrangesofworkability of concretemeasuredin accordance withIS:1195 are given below

Degree of WorkabiLity Slump (mm)

Low 25-50

Medium 50-100

High 100-150

302.11.3.Chloride content : Unless otherwise specifiedand agreed,the method of calculationand testshall be basedupon the chloride-ion contentsof all constituents and thecompositionof theconcrete.The chloride-ion contentofeachofthe constituentused in the calculation shall be oneof thefollowing:

(a) the measuredvalue

(b) the valuedeclared bythe manufacturer

(c) the maximum value where specified in the BIS for theconstituentasappropriate.

The calculated chloride contentof the concreteexpressedas thepercentageof chloride-ionby massof cementshall notexceedthe valuespecified in Clause302.6.5.

302.11.4. Density of fresh concrete : Where minimumdensity of fresh concreteis specified, the meanof any fourconsecutive samples shallnot be less than the specifiedvalueand anyindividual sample result shall not beless than97.5 percentof the specifiedvalue.

302.11.5.Densityof hardened concrete: Where minimumdensityof hardened concreteis specified,the meanof any fourconsecutivesamplesshall not less than thespecifiedvalueand

17

IRC : 2 1-2000

any individual sample result shall not beless than97.5 percent

of the specifiedvalue.

302 12 Storageof Materials

All efforts shall be madeto store the materialsin properplaces so asto preventtheirdeteriorationor intrUsion by foreignmatterand to ensuretheirsatisfactoryquality and fitnessfor thework. The storagespace shall also permit easy inspection,removaland re-storageofthe materials.All such materials, eventhough stored in approved storage, shall be subjectedtoacceptance test prior totheir use.

303.1.grades shall

303. BASIC PERMISSIBLE STRESSES

The permissiblestressesfor concrete ofdifferentbe as indicated in Table 9.

TabLe 9. Propertiesand Basic Permissible Stresses ofConcrete

Properties/permissiblestresses

M50

M55

M60

1. Modulus of elasticityEc~designvalue (GPa) 26 27.5 29 30.5 31.5 32.5 33.5 35 36 37

2. Permissible directcompressivestresses(MPa) ~, allowable 3.75 5 6.25 7.5 8.75 10 11.25 12.5 13.75 15

3. Permissible flexuralcompressive stresses(MPa) ~, allowable. 5 6.67 8.33 10 11.67 13.33 15 16.67 18.3 20

Notes:(I) Forcalculatingstressesin section,a modularratio (E,/E) of 10may be adopted.

(2) For designspecificationsof shear,bondlanchorageseeClause304.6 and304.7.

ConcretegradeMMMMMMM15 20 25 30 35 40 45

18

IRC : 21-2000

303.2. PermissibleTensileand CompressiveStressesin SteelReinforcement

303.2.1. Permissibletensileandcompressivestressesinsteel reinforcement shall not exceedthosegiven in Table 10.

Table 10. PermissIble StressesIn ReinforcingBars

Bar grade Type of stressin steelreinforcement

Pein

rmissible stressMPa

1 2 3

Fe 240 Tensionin flexure, shearor 125Fe 415 combinedbending 200Fe 500 240

Fe 240 Direct compression 115Fe 415 170Fe 500 . 205

Fe 240 Tension in helical 95Fe 415 reinforcement 95Fe 500 95

303.3. The basic permissibl~e~tènsilestressesin plainconcreteelementsshall not exceedthosegiven in Table 11.

Table 11

Concretegrade MPermissibletensile stresses

15 M 20 M 25 M 30 and above

MPa 0.40 0.53 0.61 0.67

In case of concrete memberscast in one lift with noconstructionjoints or when special precautionsare taken forsurfacepreparationof joints like useof wet sandblasting orsurfaceretarders,the basic values given in Table 11 can bepermittedto be increasedbut in no caseshall theseexceed1.25times the basicvalue given in Table 11.

19

IRC: 21-2000

303.4. Control of Cracking in Concrete

303.4.1. The requirementof crack control at the tensileface of reinforced concrete components undersustainedloadsshall be deemedto have been satisfied, provided the followingdetailing criteria are met

(i) For slabs,thediameterandspacingof reinforcingbar shall notexceed25 mm and 150 mm respectively.

(ii) For beams,including top and bottom flanges in rectangularvoided slab and box beamsand for solid slabs in solid slabbalanced cantilever bridgesthe diameter and spacing ofreinforcingbarshall not exceed32 mmand150 mmrespectively.

(iii) For columns thediameterand spacingof reinforcingbar shallnot exceed32 mm and300 mm respectively.

303.4.2.In specialcases,where the detailingcriteriastatedin Clause303.4.1 are not possible to beachieved,the~designsshall be checked for crack width limitations as specified inAppendix- I.

303.5. PermIssible Stresses under variousCombinations of Loads and Forces

303.5.1. The permissible stresses given in Clause303shall not be exceededfor combination I of Clause 202.3 ofIRC:6. The permissible increasefor other combinationsshallconform to Clause203 of IRC:6.

304. GENERAL DESIGN REQUIREMENTS

304.1. General

304.1.1. Various stressesthat are likely tooccur in anyplain and reinforced concretestructure, under the worstcombination of loads and forces, specified in IRC:6 shall beprovidedfor in accordance with accepteddesignand construction

20

IRC : 21-2000

procedureandin conformity with the fundamental principlesofmechanicswithout exceedingthe limits of stresses specifiedinClause303.

304.1.2. Thedetailingofreinforcementin all componentsshall be such as to ensure satisfactoryplacementand goodcompactionof concreteall around in thecomponentswith dueconsiderationbeinggivento theconstruction techniques adopted.

304.2. Basisof Design

304.2.1. The strengthof a reinforced concrete structuralmember may be assessed by commonly employedelastic theoryand it may be assumed that

(i) the modulus of elasticity of steel is 200 GP~unlessotherwise

determinedby tests.

(ii) the modularratio hasthe values given in Table 9 (note 1) and

(iii) unlessotherwisepermitted, the tensile strengthof concreteisignored.

304.2.2. In plain concretestructures,tension upto limitsspecifiedin Table 11 may be permitted.

304.3. Cover

304.3.1. The minimum clear cover to anyreinforcementbar, closest to the concretesurface, shallbe 40 mm.

304.3.2. Increased minimum cover thicknessof 50 mmshall bc proyided when concretemembersare exposed to severeconditions of exposureas mentioned in note(1) of Table 5except thatfor the conditionof alternate wetting and ~dryingandin capeof foundationswherethe minimum clearcovershall be75 mm.

304.3.3. The abovecovermay bereducedby 5 mm for

21

IRC 21-2000

factory made precastproducts with higher level of qualityassurance.

304.4. Bar Sizes

304.4.1. The maximumsizeof reinforcementshallbe 40mm diameteror a sectionofequivalentarea,unlessa biggersizeis permittedby the competentauthority.

304.4,2. The diameterof any reinforcing bar, includingtransverseties, helicals,stirrupsandall secondaryreinforcement,shall generallybe nOt less than 8 mm.

304.4.3. The diameterof longitudinal reinforcingbars incolumnsshall not be less than 12 mm.

30444. The diameterof reinforcementin slabsshallbelimited to one-tenththedepthof slab; andthe diameterof shearreinforcement in beam-webs,including crankedbars, if any,shall be limited to one-eighththe thicknessof the web.

304.5. DistanceBetweenBars

304.5.1. The horizontal distancebetweentwo parallelreinforcing bars shall not be less than the greatest of thefollowing threedimensions

(i) the diameterof the bar - if the diametersare equal;

(ii) the diameterof the largestbar - if thediametersare unequal;and

(iii) 10 mmmore thanthenominalsizeof thecoarseaggregateusedin concrete.

Note : In order to comply with theprovisionsof this sub-clause,the sizeof the coarse aggregatefor the concrete around congestedreinforcementmay be reduced.This doesnot precludethe useoflargesize aggregatewherethe reinforcementis not congested.

22

IRC: 21-2000

304.5.2. Sufficientspaceshall be leftbetweengroupsofbars to enable thevibrator to be inserted.

304.5.3. The minimum vertical distancebetweentwohorizontal main reinforcing bars shall be12 mm or themaximumdiameterofthe coarseaggregateorthe maximum sizeof the bar, whicheveris greater.

304.5.4. When contactof bars along the laplengthcannot beavoided,suchbarsshall preferably be grouped in thevertical plane. Inno case,however, shall there bemore thanthree bars in contact. The vertical and horizontaldistancesspecified in the Clauses304.5.1, 304.5.2 and 304.5.3 shall bemaintainedbetweenany such group and an adjacentgroup orbar.

304.5.5. Subject to satisfyingcrack control criteria asgiven in Clause303.4the pitchof bars orwiresof main tensilereinforcementin slabs shall not exceed300 mm or twice theeffective depthof the slab whicheveris smaller.

304.5.6. All mesh reinforcement shall beof suchdimensions as willenablethe coarsestmaterial in the concreteto passeasilythrough themeshesof suchreinforcement.

304.6. Bond, Anchorage,Splice

304.6.1. To prevent bond failure, design tension orcompressionin any reinforcing bar at any sectionof anelementshall be developedon eachside ofthe section byan appropriateanchoragelength conforming to provisions givenin Clause304.6.2orby a specialanchoringdeviceorby a combinationofboth.

Provided this is done, localbond stresses may beignored.

23

IRC 21-2000

304.6.2. Anchorage length

304.6.2.1. lermlssiblebasicvalue-b Thelengthrequiredfor developingfull strengthof bar with straight endsin tensionat anchorageto keep theaveragebond stresses within allowablelimits is termed as the basic permissible valueof anchoragelength lo = n

The valuesof n are givenin Table 12 varying with

gradeof reinforcementgrade of concreteandbondingzone

Table 12. Values of lo as Multiplies of 4)

n =_lo/4)

Concretegrade M 20 M 25 M 30 M 35 M 40and above

Bar grade

Bonding zoneI(favourable)

Fe 500

Fe 415

Fe 240

66 56 48 42 42

55 46 40 35 3565 60 55 50 50

Bonding zone11(unfavourable)

Fe 500

Fe 415

Fe 240

1.4 times the values given forbondingzoneI

Bonding zone II (unfavourable)shall apply to horizontal orbarbentless than 450 and so located that morethan300 mm offresh concreteis cast below thebar, Fig. 1.

Bonding zone I(favourable)shall apply tobarnot locatedin bonding zone II.

304.6.2.2. Design value‘d for barsin tension

(1) Bars in tension shallbe developed either bya design anchoragelength with straight ends or with end hooks of the shape

24

IRC: 21-2000o~icr’o.~oc co~c~~n~a

____ 4Ii

R 114 SONDING ZONC L —FAVOU~ABl.E

_____ I~OOj-~ø. DIRICTION CONCREflNO

1~300mm’

BAR IN nONDING ZONI U UNFAVOURABLE

Fig. 1. Bonding zones

specified in Fig. 2. Forbars of grade Fe 240 anchoragewithstraight endsis not permitted.

(2) Bars developing for full tensilestrength shall have designanchoragelength.

= Ia) = (1,10

aI = I for bars with straight ends= 0.7* for barswith end hooks

*applies when cover in the planeof curvature andtransverseto it isnot less than 34). With less cover,a~= I applies.

(3) At sections where reinforcement area in excessof designrequirement(A, � A,d) has been provided a reduced length -

td = ~c2 =

shall be provided, where

~x AJA withcc>l/32 s s 2—

cc (factor for excessreinforcement)= AjA

= areaof bars actuallyprovided

= areaof bars required at full permissiblestress

(4) ‘d adoptedshall at least be equal toIa, mm., in all cases- id> 1, minimum

25

IRC : 21-2000

dl > 6 4 FOR.Fe 415/500BARS~ 4 4 FOR Fe 240 BARS

24

BARS WITH END HOOKS

d2 ~ 20 4 FOR Fe 415/500 BARS

> 15 4 FOR Fe 240 BARSBENT UP (DOWN) BARS

Fig. 2. Bend diameterfor hooksand bentup (down) bars

Id

r~Io~

di

26

IRC: 21-2000for bars with straight for bars with end hooks

ends

1, mm. = 12 4) or 300 mm 1, mm = 6 4) or 150 mm

Whicheveris greater. Whicheveris greater,

304.6.2.3. DesIgnvalue for bars in compression: Barsin compression shall be developed byan anchorage length withstraight ends, end hooksbeingdeemed not effective and valueof ‘d shall be sameas that specifiedfor bars in tension in Clause304.6.2.2.

304.6.3. Anchorageofflexural reinforcement in beamsand slabs : The provisions of the clause shallillustrate,supplement and modify the general requirements given in’Clause 304.6.2.

304.6.3.1.Anchorageof bars over bearings : Anchoragelength of bottom bars may be reduced to 2/3 at end or atintermediatesimple supportto caterfor the favourable effectsoftransverse compression. Anchorage length shall bemeasuredfrom the inner (span) faceof the support,Fig. 3.

7 1.ONSITUO0~KA~~

ANCHORAGE OVLR SCARING DIRECT SUPPOWF

LONOIT(~IP4A~.SIAM

IEARiNO~

ANChORAGE OVER SEARING : INDiRECT SUPPORT

Fig. 3. Anchorage over bearings

27

JRC 21-2000

For indirect support full Id shall be provided.

304.6.3.2.Anchorageof bars in tensIon zone : Bars maybe curtailedand anchoredin the tension zoneofa beam orslabwhen the following conditions are satisfied

(1) The curtailment and anchorage is done according to Fig. 4.

(2) Curtailmentof barsis optimally staggeredand the areaof barscurtailedsatisfies the limits on continuing bars laid down inClause 305.7.

(3) The endsof curtailed barsare bent up at a very flat angleoverthe anchoragelength to yield a bigger cover asshown in Fig.4.

Id DESIGN ANCHORAGE LENGTHa, to > I, MIN, ~ato

FULL STRENGTH ANCHORAGE LENGTHa, to> to Mis.

Id, a,lai>IoMin,REDUCED ANCHORAGE LENGTHFOR EXCESS REINFORCEMENT

[4 • REDUCED ANCHORAGE LENGTHFOR SUPPORT

V = SHIFTFORCURTAILMENTLINEr — A ~ORHAN

1.5 100 01.10 V0 = SHiFT AT END SUPPORT

— 011TH Or ~ ,,,,,,~j o 50 1 0

DEThIL AT A AT BOll STOP

Fig. 4. Curtailment and anchorage of bars in tension zone

304.6.3.3.Anchorageof bars bent up (down) : Bars notrequiredfor shearresistanceshall havean anchoragelength idmeasured from the pointof bending.

Bars required forshearresistance will be continued to theopposite faceofthe beamlslab andanchoredwith a lengthof0.6

28

LRC : 21-2000

BENT — DOWN OF SHEAR ANCHOREO IN THE ZONEOF TENSILE STRESSES IN THE CONCRETE.

BAR USED AS SHEAR REINFORCEMENT0.6 Id.

L~b. ji~i~cT~fBAR NOT USED AS SHEAR REINFORCEMENT

Fig. 5. Anchorage of bars bent up (down)

Id1 in compression zone and1,3 Id1 in tension zone parallel tofaceof the beam,see Fig. 5.

304.6.4. The shear stirrups shall be deemed adequatelyanchored when the rcquirements illustratedin Fig. 6 are metwith.

304.6.5.Specialanchoringdevice : When theendsof asecondary reinforcement, such as stirrups or binders, areanchoredin the compression zone; completeanchorageshallbenotwithstanding any of the other provisionsof this Code,deemedto have beenprovidedwhen the endis bent round a barof at leastits own diameterthroughan angleof at least900 and

BENT — UP OF SHEAR . ANCHORED IN THE ZONEDr COMPRESSNE STRESSES IN THE CONCRETE.

(BAR USED AS SHEAR REINFORCEMENT)

29

IRC: 21-2000

CLOSING IN TP4E COMPRESSION ZONE

continuedbeyondthe endof thecurve for at leasteight times thediameteror alternativelythroughan angleof at least1800 andcontinuedbeyondtheend of thecurvefor at leastfour tines thediameterof the bar.

304.6.6. Splices

304.6.6.1, Splicesof reinforcement shall be formed by

(1) lapsof barswith straight endsorbarswith endhooks

(2) weldedjoints

(3) joi~ttswith mechanicaldevices

.4

‘oil’ IIII ~os.iiusmuIt

‘oul~

CLOSING IN THE ‘TtNgLEZONE

RE INFOI~CLMENTOF THE CONNEtTINOSLAS

CLOSING WITH T-BEAMS IN THEAREA OF THE SLAG

50TH FO~ COMPRESSION ANDTENSION ZONE

Fig. 6, Shape of stirrups

30

IRC: 21-2000

304.6.6.2. Lap splicesof bars in tension

(1) ]..~apsplicesof bars in tension shall be longitudinallystaggeredas far as practicable. Thelongitudinal andtransverse spacingshall conform to Fig. 7.

H ‘3 ~

(2) Areaof bars spliced at any sectionshall not exceedthe following proportions p in relation to the totalareaof bars provided at thesection:

bar grade pFe 500/415

Fe 240

In exceptionalcases,100 per cent lapping atasectionmaybe pem’iitted for deformed barsonly, subject tospecialprecautionsandsupplementalspecifications.

CENTRESOF SPUCES

.1

Lr

có

Edgedistance

Fig. 7. Longitudinal staggering and transverse distance of reinforcingbarsin the region of lap splices(dimensionsin mm)

50 per cent25 per cent

31

1RC : 2 1-2000

304.6.6,3.Lap length Ia

(1) the length of lap of bars in tensionwith straight endor with endhooksshall he la Ki~wherethe valueof K varies with p as given below:

p % ~ 25% > 25% > 40% 50%

~ 40% ~ 50%K= 1,4 1,6 18 2.2

The values of K can he reducedby 20 per cent whena,

> 10 ~ and/orb > 5 ~.

Where a = axial spacingof the barh = edge distanceof bar shown in Fig. 7.

(2) the length of lap of bars in compressionshall be equalto I~.Hooksshall not be considered effective.

304.6.6.4. Transverse reinforcementat lap splicesMinimum reinforcement in the form of stirrups shall heprovtded over thelengthconformingto the requirements shownin Fig. 8.

ri— -

¶ ISTiRRUP

—1a

J

Fig. 8. Longitudinal section

32

IRC : 2 1-2000

304.6.6.5. Welded joints may be used subject to thefollowing

(a) Welding of Fe 240 grade bars conforming to IS:432 shall bepermitted. Welding of other grades of bars including Fe 500/415 grade bars conforming to IS: 1786 shall in general beprohibited except in special cases mentioned in (h) below,

(h) Welding shall conform to 15:2751 except as provided herein.

(c) Wclding may be carriedout by metal arc welding process. Oxy-acetylene welding shall not be permissible. Any other processmay be used subject to the approval of the engineer andnecessary additional requirements to ensure satisfactory jointperformance. Precautions on over heating, choice of electrode,selection of correct current in arc welding etc., should hestrictly observed.

(d) All bars shall be butt welded except for smaller diameter basswith diameter of less than 20 mm which may be lap welded.Single-V or Double-V butt joints may generally be used, Forvertical bars single bevel or double bevel butt joints may heused,

(e) Welded joints shall be located well away from bends and notless than twice the bar diameteraway from a bend,

(1) Generally, shop welding in controlled conditions is to bepreferred, where feasible, Site welding where necessary shall,however, be permitted when the facilities, equipment, process,consumables, operators, welding procedure are adequate toproduce and maintain uniform quality at par with that attainablein shop weldingto the satisfaction of the engineer.

(g) Joint welding procedures which are to be employed shallinvariably be established by a procedure specification and shallbe qualified prior to use by tests as prescribed in 15:2751. Allwelders and welding operators to be employed shall have to bequalified by tests prescribed in IS:2751, Inspection of weldsshall conform to IS:822 and destructive or non-destructivetesting may be undertaken when deemed necessary, .Ioints with

33

IRC: 21-2000

weld defects detected by visualinspection or dimensionalinspectionshall not be accepted.

(h) As exception to the provisionof(a) above,barsof grades otherthan Fe 240 grade including special weldinggrade of Fe 500/415 grade barsconforming to lS:1786, for which necessarychemical analysishas been secured and thecarbon equivalentcalculatedfrom the chemical composition using the formula

Mn Cr+Mo+V Ni+CuCE. = C + + +

6 5 15

is 0.4, or less, maybe permitted tobe welded, The method ofwelding shall conform to supplemental specifications tothesatisfactionof the engineer and shall besubject to suitablequalification tests.

304.6.6.6. At the weldedjoints complying with Clause304.6.6.5, while 100 per cent of the cross sectional area may betaken into account as effective for Fe 240 grade bars conformingto ES:432, only 80 per cent of such area in respect of bars otherthan mild steel including Fe 500/415 grade bars conforming toIS:1786 shall be considered as effective.

304,6.6,7. Bars may be joined with mechanical devicese.g. by special grade steel sleeves swaged on to bars in end toend contact or by screwed couplers.

The effectiveness of such joints shall invariably be provedby static and fatigue strength tests. Patented system~~swithproven use shall only he permitted to be used on production oftest results showing the adequacy of the device to the satisfactionof the competent authority.

A mechanical joint including its connecting elements shalldevelop in tension or compression at least 125 per cent of thecharacteristic strength j.

34

IRC : 21-2000

304.7. Shear and Torsion

304.7.1. Shear

3043.1.1. Shear stress

3047.l.l,l. The design shear stress t at any cross sectionof beams or slabs of uniform depth shall be calculated by theequation

V

b~d

Where V = the design shearacrossthe sectionb = breadthof the member,which for flanged sections

shall be takenas thebreadthof the web, andd = effective depth of thesection

Note : For obtainingthe maximum shearstress,the section at a distanceequal to effective depth fromthe face of the support shall bechecked and theshear reinforcement calculatedat the section shallbe continuedup to the support.

304.7,1.1.2. In case of a beams or slabs of varying depth,the equation shall be modified as

V± Mtan/3d

b,d

Where r, V, b andd are the sameas in 304,7,1,1,1

M = bending moment at the section,due to loadposition corresponding to shear V

/3 = angle between the top and thebottom edgesofthe beam atthat section,

The negative sign in the formula applies when the bendingmoment M increases numerically in the same direction as theeffective depth d increases, and the positive sign when themoment decreases numerically in this direction.

35

IRC : 21—2000304712. Maximum permissible shear stress -r

maxWhenshear reinforcementis provided the sheart in beams shallnot exceed stresstmax~given in Table 12k

For slabs,r shall not exceedhalf the valueof t , givenrflLLN

in Table 12k

Table 1 2A. Maximum Shear Stress, t , N/mm2Ut”

Concrete Grade M20 M25 M30 M35 M40 andabove

‘max, N/mm1 1.8 1.9 2.2 2.3 2.5

304713. Designshear strengthof concrete

3047.1.3,1. The peniiissihleshearstress -r~in concreteinbeams withoutshearreinfocementis given in Table I 2B.

304.7.1.3 . For solid slabsthe permissibleshearin concreteshall be K.-rwhere.Khasthevaluesgiven in Table 12C.

3047133!. Shear strength ofmembersunder axialcompression: For memberssubjectedto axial compressionP,tile permissible shearstress in concreteç given in Table I 2B,shall be multiplied by the following factor:

5Pd — 1 ~- but not exceeding 1.5

Afg

Where P axial compressive force in Newtons,= gross area of the concrete section in mnY, and

1ck characteristic compressive strength of concrete

36

ERG : 21-2000

Table 12W Permissible Shear Stress in Concrete

100 Alxi

Permissible Shear Siress in Concrete, t N/mm2Grade of Concrete

M20 M25 M30 M35 M40(1) (2) (3) ~4) (5) (6)

0.150.250,500... 75’1.001.251 .501.752..002.252.502.75

3, on and above

0.18 0.19 0.20 0.20 0.200.22 0.23 0.23 0.23 0.230.30 0.31 0.31 0.31 0.320.35 0.36 .. 0.37 0.37 0.380.39 0.40 0.41 0.42 0.420,42 0.44 0.45 0.45 0.460.. 45 t).46 0.48 0.49 0.490.47 0.49 0.50 0.52 0.520.49 0.51 0.53 0.54 0.550.51 0.53 0.55 0.56 0.570.51 0.55 0.57 0.58 0.600.51 056 . 0.58 0.60 0.620.5 I 0.57 0.60 0.62 0.63

Nute~, ‘A” is that area of longitudinal tension reinforcement whichcontinues at least one effective depth beyond the section beingconsidered except at supports where the full area of tensionreinforcement may be used provided the detailing conforms toClause 304.6.

Table 12G. Valuesof K for Solid Slabs

Ovenill 300 or 275 250 225 200 175 150 ordepth of more lessslab, mmK 1,00 1.05 1.10 1.15 1.20 1.25 1.30

304.7. I .4. Memberswith shear reinforcement: When ‘rexceeds‘r~given in ‘Table 1213, shear reinforcementshall he

provided in any of th.e thRowing forms

‘37

IRC: 21-2000

a) Vertical stirrups,

b) Bent-up bars along with stirrups, and

c) Inclined stirrups

Where bent up bars are provided, their contributiontowards shearresistanceshall not be more thanhalf that of thetotal shear~reinforcement,

Shear reinforcementshall be provided to carry a shearVs V - t.bd. to be calculated asbelow

vsA

5 — .— --

.d. (Sina .f Cosca,)Where 44 = total cross-sectionalarea nf stirrup legs or bent-up bars

within a distance s,

s = spacing of the stirrups or bent-up bars along thelength of the member,

b breadth of the member which for flanged beams, shallbe taken as the breadth of the web,

a = permissible tensile stress in shear reinforcement

a = angle between the inclined stirrup or bent tip bar andthe axis of the member, not less than 45°,and

d = the effective depth

Note: Where more than one type of shear reinforcement is used toreinforce the same portion of the beam, the total shear resistanceshall be computed as the sum of the resistance for the various typesseparately. The areas of the stirrups shall not be less than theminimum specified in 304.7.1.5.

304.7.1,5. Minimum shear reinforcement forbeamsWhen -r is less than t~given in Table 12B, minimum shearrein fbrcementfor beamsshall he provided in accordancewith

38

IRC: 2 1-2000

the ‘ftillow ingA 0.4

Pu mm, .‘:::........... . f C 415 MPab.s 0.87 1

304.7.1 .6. Maximum spacingof stirrups shall be limited toone—halfthe depth of the beam subjectto a maximum of 300rum. Stirrups shall passround, or otherwisehe securedto theappropriate longitudinaltensile reinforcement. The ends ofstirrupsshall be adequately anchoredin the compression zoneinaccordancewith Clause304,6,4. Where forpractical purposesitis’tbund necessaryto anchor the endsof tile stirrups in thetensile zone. full anchoragelength in accordancewith Clause304.62.2shall be provided.

304.7,,1 .7. Bent—up barsshall b.c carried through a depthofat least equal to the lever arm of the resistingmoment andadequately anchoredin accordancewith Clause304.6.3.3. Thespacingof th~bent—upbar measuredat thelevel of neutralaxisand in the direction of longitudinal axis of the beam shall notexceedthree-quarter the effective depthof the beam.

304.7.2. Torsion

304.7.2.1. General : In structures:where torsionis requiredto maintain equilibrium,menibersshall be designed fortorsion.However,for such indeterminatestructureswheretorsion canbeeliminatedby releasingredundant restraints,no specific designfor torsion is necessaryprovidedtorsionalstiffhessis neglectedin the calculation of internal forces.. .Adeqi.iate control of anytorsionalcracking is providedby the shear reinforcement as perClause304.7.1.

Torsional reinforcementis not calculated separately fbrtorsion alone. Instead the total longitudinal reinforcement isdetermined fora fictitious bendingmomentwhich is a function

39

mc : 21-2000

of actual bendingmoment and torsion; similarly webreinforcement isdetermined fora fictitious shearwhich is afunctionof actual shearandtorsion.

The designrules shall apply to beamsofsolid rectangularcross-section.However, theseclauses may also be applied toflanged beamsby substituting bw for b, in which casethey aregenerally conservative.

304.7.2.2. CrItical section: Sections locatedless thanadistanced, from the faceof the support may be designedfor thesametorsion as computed at a distance d, where d is theeflbctive depth.

304.7.2.3. EquIvalent shear

304.7.2.3.1.Equivalentshear, V~,shall becalculatedfrom

the formula:

Where V~ - equivaLentshear,

V = shear,

T = torsional moment,andb - breadthofbeam

The equivalentnominal shearstress,;, in this caseshallbe calculated as given in 304.7.1.1.1.except for substitutingVby Y,. The values of; shallnot exceedthe valuesoft~ givenin Table 12A.

304.7.2.3.2.If the equivalentshear stress; does notexceed;,given in Table 128, minimum shearreinforcementshall beprovidedas specifiedin 304.7.1.5.

304.7.2.3.3.If; exceedsthose givenin Table 128, both

40

IRC 21-2000

longitudinal and transverse reinforcement shallbe provided inaccordancewith 304.7.2.4.

304.7.2.4. Reinforcement in members subjected totorsion

304.7.2.4.i. Reinforcement for torsion, when requiredshall consist of longitudinal and transversereinforcement.

304.7.2.4.2.Longitudinal reinforcement The longitudinalreinforcementshall be designedto resist an equivalent bendingmoment,Ml, given by

Ml= M + Me

Where Al = bendingmoment at the cross section,and

T(1+Dfb)Al = -‘——“ ¶ whereT is the torsional moment,

1.7

1) = the overall depth of the beam

h = breadth ofthe beam,

304.7.2.4.2.!.If the numerical value of Al as definedin3047,2.4.2. exceedsthe numerical value of the mementAl,longitudinal reinforcementshall he provided on the flexuralcompressionface, suchthat the beamcan also withstand anequivalent momentMt1 given by Me,=M,-M, the moment Me,being takenasacting in the opposite senseto the momentM.

304.7.2.4.3. Transverse reinforcement Two leggedclosedhoops enclosingthe cornerlongitudinal barsshall havean areaof cross sectionA~given by

T.s V.sA,,, = —‘—-—-‘ + —‘--—‘—‘ ,hutbd~a, 2.5d1a,

41

IRC 21—2000

the total transverse reinforcementshall not he less than

(re r~)b.s

Where 7’ :.:::: torsional moment

V = shear force

4, crosssectional area of barforming the closedhoop

= spacingof the stirrup reinforcement

centre to centre distancebetweencorner bars inthe

directionof the widthd centre to centre distancebetweencorner bars in the

directionof the depth

Zr breadthof the member

a = permissibletensile stressin shear reinforcement

r equivalentshearstres.s asspecified in 304.7.2.3.I , and

r :::..: shear strengthof the concreteas specified in Table12B

304.7,2.5.Distribution of torsion reinforcement Whena memberis designedfor torsion, torsionreinforcementshall beprovided as below

(a) The transversereinforcement shallbe rectangularclosedstirrupsplacedperpendicularto the axis of the member,The spacingof

the surrops shall not exceed the smallerof or 300 mm.

Where~ and Y1 are respectivelythe short and longdimensions

of the member.

(h) In all casesthereshall be at least one longitudinal bar in eachcorner of the stirrups. The diameter of these longitudinal barsshall not be less than the diameterof the stirrups or 12 mmwhichever is greater.

42

IRC :21-2000

304.8. Moment of inertia

304.8.1. Moment of inertia for calculating relativestiffness : To determinethe relative stiffness of membersofstatically indeterminatestructures,themomentof inertiamay hecalculatedby considering

(i) the. entire concrete section,ignoring the reinforcement,or

(ii) the entire concretesection, including the reinforcementon thebasis of the modular ratio, or

(iii) the compressionareaof the concretesection,combinedwith thereinforcement,on the basis of the modularratio,

Whichevermethod is adoptedfor the beams,the samemethodshould be usedfor the columns.

304.8.2. Moment of inertia for calculating deflection:Appropriatevaluesof momentof inertia may be used.

304.9. Temperature and Shrinkage Effects

304.9.1. The valuesof co-efficientsof thermal expansionand shrinkage of concrete shall be taken as follows

(i) Thermal expansionFor reinforced concrete 1] 7 x lQ’~per degreecelsius

For plain concrete 108 x l0’~per degreecelsius

(ii) Shrinkage 2 x lO’~

304.9.2. Every simply supportedspanshall beprovidedwith meansto permit both rotation and longitudinal expansioncausedby the design loadsand forces,

305. BEAMS AND SLABS

305.1. For bridges having beam and slab type ofsuper-structurethenumberof longitudinalsshallnot be less than

43

1W : 21-2000

three, except for single-lane bridges and pedestrian bridges.

305.2. The minimum thickness of the deck slabincluding that at the tip of the cantilever shall be 200 mm.However, reduction in the thickness of the slab upto a maximumof’ so mm may be permitted at the cantilever tip subject tosatisfactory detailing. The thickness of the webs shall not be lessthan 250 mm.

305.3. Cross girder monolithic with the deck slab shallbe provided at the bearings. Intermediate cross-girders shall beprovided depending on design requirements. The thickness ofthe cross girders shall not be less than the minimum webthickness of the main longitudinal girder. The depth of the crossgirders at bearings shall be suitably adjusted to allow access fixproper inspection of bearings and to ficilitate positioning ofjacks for future lifting up of the super-structure.

305.4. Effective Span

305.4.1. In the case of flee supports on line bearings, theeffective span lo shall be the distance from the centre to centreof the bearings.

305.4.2. In the case of a free support, not on linebearings, the effective span lo shall be 1, + d or 1 whichever issmaller,

Where 1 — distance betvieen cenfl of supportsa clear span

J effective depth of beam or slab.

305.4.3. in the case of restraint at the support, the effectivespan may be taken as equal to the clear distance between thefaces of the supports.

44

!RC: 21-2000

3055. Effective Depth

305.5.1 The effectivedepth of a beamor slab shallbethe depthfrom edgeof the compressimisectionto the centroidof Ihe tensionreinforcement.

305.5.2. Wherehaunchesareprovided,no portion of thehaunch lying below a plane which makesa slope of 1:3, asshown in Fig. 9 shall be consideredas adding to the effectivedepth.

However, for calculating themomentsin a haunchedslabor beam, theactual varjation of momentof inertia in the spanshall be considered.

305.53. Where memberthicknessvaries, no portion of themember lying below or above a planewhich makesa slopeof

I :3 with the horizontal shall be consideredas adding to theeffective depth.

TAN�~ ~f3

Fig. 9. Portion below line AB to be ignored for effective depth

45

IRC 21-2000

305.6 Compression Reinforcementin Beams andSlabs

(i) Beams When in a beam,part orall of the mainreinforcement is requiredto resist compression, links or tiesatlea,st one quarter of the size of the largest compression barshould be providedat a niaximwii spacingof I 2—times the sizeof the smallest compressionbar. Links should he so arrangedthat every con-icr and alternate bar in an outer layer ofreinforcement is supportedby a link passing round the barhavingan includedangleof not more than 1350. All other barswithin a compressionzone shall he within 150 mm of arestrainedbar,

(ii) Slabs When the designed percentageofreinforcementin the compressionfaceof a slab exceeds1 percent,links of atleast6 nini or one quarter the sizeof the largestcompression bar should he provided for a depth of 200 mmthrough the thicknessof the slab.. If the thicknessof the slab is.insufficient to accomniodatethen it should be tied with thetension bar. Thespacingof theselinks should not exceedtwicethememberthicknessin eitherof the two principal directions,ofthememberor 400 mn-i and in the directionof the compressionforce. not greater than16 times the barsize.

305.7. Curtailment of Bars

305.7.1, To prevent large changes in the monient ofresistancethe points a.t which the barsare curtailed shall besuitably spread.

305.7.2. In simply supportedspans,at least33 per centof the steel requiredto resist the maximum bending momentshall he carriedover the supportsalong the tensionside of thebeam andin the caseof slab this shall not be less than 50 per

46

[RC 2 I —2000

cent. For a spancontinuous beyonda support, at least 25 percent of thesteelrequiredto resistthemaximumpositivebendin~moment shall be similarly carriedover thesupport both in caseof beam orslab.

3058. Cnned or SlopedReinforcement

\Vhcre the alignment of the reinforcementdeviatesfromthe normal to the planeof bending,asin the caseof a beamwithcarved or sloped soffit, only the areaof the reinforcementeffective in thedirectionnormal to the planeof bendingshall beconsidered.

305.9. LateralSupport in Beams

in flanged beams with length between adequate lateralrestraintsgreaterthai-i thirty times the width of the compressionflange, the permissiblecompressive stressin concrete shallbe

loreduced by a factor equal to l.5--~ where ‘? is theunsupportedlengthand b is thewidth ofthe compression flangeof the beam.

305. I 0. Shrinkage and Temperature Reinforcement

All thces of reinforced concrete member either fullyexposedor lying within a depth of 500 mm below thelevel of

perennial suhniergenceunder water, soil or soil-soil-watersystem shall be reinforcedfor shrinkageand temperature.This.reinfbrcenientshall be providedin two directionsat right anglesto each otherin a planeparallelto thesurfaceunderconsiderationand shall he spacedat 300 mm maximum with 250 mm2 of steelareaper metre in eachdirection forall gradesof reinforcement.

Reinforcing bars placed near thesurface of a memberin eachdirection to carry other loads may’ be considered as fully

47

IRC 21—2000

effectivein providingreinforcementfor shrinkageandtemperaturee.g. main longitudinal reinfbreement,distributionreinforcementin slabs, web reinforcement in beamsand ties or helieals incolumns.

305.11. Dispersion of Live Loads on LongitudinalBeams

The dispersionof the live load along the span lengththrough the wearing coat. deckslab and filling shall not heconsidered.

305.12. Distribution of Live Loads on LongitudinalBeams

305..12. 1. When longitudinal beamsare connected togetherby transversememberslike deckslab, crossgirders,diaphragmssoffit slab, etc., thedistribution of bending momentsbetweenlongitudinals shall be calculated by one of the followingmethods

(i ) Finding the reactionson the longitudinalsassumingthe supportsol’the deckslab as unyielding.This methodis applicablewherethereare only two longitudinals ~ ith no soflit slah~

(d) Distributing the loads between longitudinals by (‘ourbon’smethod, strictlywithin its limitation, i.e., when theeffectivewidth of the deck is less than half the span and when thestifihessof the cross girdersis ~‘erymuch greater thanthat ofthe longitudinals; and

(iii) Distributing the loads bet~~een longitudinals by aim rat~onalmethodof grid analysis,e.g., the method of harmonicanalysisas given by Hendry andJaegeror Morice and Little’s versionof the isotropic plate theory of Guyon and Massonet,etc.

305.12.2. In calculating theshear forceon sectionsoflongitudinal beams,wheel loadsor ti-ack loadsshall be allocated

48

1RC: 2 1-2000

to. respective longitudinal beams by any rational method.Alternatively, the following methodshall be followed

(i) For loads at or within 5.5. metresof either supports

The reaction on the longitudinal beams shall be the greater ofthe resultsobtainedby

(a) assuming the deckslab simply supported or continuousas the case mayhe with the supportsbeing assumedas

unyielding and

(b) following one of the three methodsgiven in (‘lause305,12.1 for the distribution of bending moments.

(ii) For loads more than 5.5 metresaway from eithersupports

Distribution of the loads betweenthe longitudinals for thepurpose of finding shearingforces shall be assumed tohe thesameas for bending moments given inClause 305.12.1.

305.13. Dispersion of Live Loads on TransverseBeams

Dispersionof live loads alongthe spanlength throughthewearing coat, deckslab aM filling shall not be considered.

30f..14. Distribution of Live Loads on IntermediateTransverse Floor Beams

1)istribution of loadsbetweenlongitudinal beams for thepurposeof finding bendingmonentsand shearsin intermediatetransversefloor bean-isshall be made by one of the methodsgiven in Clause305.12.1.

305.15.. T-Beams and L-Beams

305.15.1. For T-beams or h-beams, theslab shall be.c.ons~deredas an integral part of thebean-i if adequatebotid andshearresistanceare providedat the junctionof the slab a..iid theweb of the beam.

49

IRC 21-2000

305.15.2. In the absenceof a more accurate determination,the effective width forcompressionflangesof beamswith solidwebs andhollow box sectionsshall be taken as the following,but not exceeding theactual flangewidth.

b + 1/5 / (1-beams)

b = b + 1/10 1 (L-beams)

Where b, = effective width for compression flange

h = thickness ofthe web for beams (for1-beamand slab)or thickness ofthe webs + width of the slab betweenwebs (for hollowbox sections).

= the distance between pointsof zero moments.(in theabsenceof elaboratecalculations,for the continuousspans, itmay be takenas 0.7 timesthe effective span).

The effective width shall be taken as constant over theentire span. In general,the effective width for tension flange

may he taken as actualvalue.

305.15 .3. if T-forni or L-form is usedonly for the purpose

of providing additional compressionarea, such asin continuous

bw

Fig. 10

0

~4bw

50

IRC 21-2000

beanisoversupports,the flangethicknessshall not be less thanone—halfthe width of tl’ie weband a total flange width not morethan four times the width of the web. For effective stresstransfer, it is desirableto splay thejunction of the web and theflangeso as to form an angleof not lessthan 110°,as shown inFig. 10.

305.15,4. Where the principal reinforcementin a slabwhich is considered as the flange of a T-beam or L—beam ~s

parallel to the i:eam,transversereinforcementshall he providedat the top of the flange. This reinforcementshall he equal tost ~ty per centof the main reinforcementof the slab at its mid—

s~nunless it is specially calculated. The length of suchreinforcing bars shall he as indicatedin Fig. 11

1~

V

.1

-~

. bw ~ ~ ~:J4

Fig. 11

51

1 RC 21—2000

305.16. Effect of Live Loads on Deck Slabs

305.16.1. TI-ic effect of concentrated loads on slabsspanningin one or two directionsor on cantileverslabs: mayhecalculatedfrom the influence fields o..f such loads or by anyother rational metimd. A value o,f 0.. 1 5 may be assumedforPoisson’s ratio.

305.16.2. The bending momentper unit width of slabcausedby concentratedloads on solid slabs spanning in onedirection or on cantilever slabs, may also be calculatedbyassessingthe width of slab that may be taken as effective inresisting thebending momentdueto theconcentratedloads. ld)rprecastslabs,ti-ic term ‘actual width of slab’ usedin this Clauseshall indicate theactual width of eachindividual precast unit.

(I) Solid slab spanning in onedirection

(I) 1~ora single concentratedload, the effective width maybecalculaltedin accordance with the followingequation

=a~(i ~

Whcrc the effective width ofslab on which the load acts,

I = the effective span as indicated inClause.305.4,

a = the distanceof the centreof gravity of theconcentratedload from the nearersupport,

= the breadth of concentration area ofthe load, i.e., thedimensionof the tyre ortrack contact area overtheroad surfaceof the slab in a directionat right angles tothe spanplus twice thethicknessof the wearing coator surface finish above the structural slab,and

a = a constant having the following values depending

upon the ratio where h is the width of the slab

52

IRC’ :21-2000

Provided that the effective width shall not exceed theactualwidth of the slab. And provided further that in easeof a

load near the unsupportededge of a slab, the effective v~idthshall i~iolcxccedthe aho~e value nor half the above value plus

the distanceof the load ironi the unsupportededge.

-~

I,

a (or sunphsupported

slat,

a forcontinuous

slab

0.1 0.40 0.40

0 2 0.80 0.800.3 1.16 1.16(4 1.48 1.44

‘(5 1.72 1.681(6 1.96 1.84

07 2.12 1.96(1.8 2.24 2.080.0 236 2.161.0 2.48 2.24

a for simpi)

supportedslab

a for

continuousslah

1.1 260 2.281.2 2.64 2.361.3 2.72 2.40

1.4 2.80 2.48

1.5 2.84 2.481.6 2,88 2.52

1.7 2.92 2.561.8 2.96 2.601.9 3.00 2.602 & 3.00 2.60

above

ii) l’or two or more concentratedloads in a line in the direct m ofthe span,the bending moment perunit width of slab shall hecalct.ilatedseparatelyfor each loadaccording toits appropriateeffective width of slab calculatedas in (I) above.

(iii) F’or tao or mme loadsnot in a line in the directionof the spanIf the effective width ofslab for one load overlaps the

effective aidth of slab for an adjacent load,the resultanteffective aid0i for the two loads equals the sum of therespective effectiveaidths for each load minus the width ofO\ crlap, providedthat the slab so designed islested for the twoloads actingseparately

53

I RC : 21 —2000

(2) Solid slab cantilever

(i) For a single concentratedload, the effective widthmay hecalculated in accordance withthe. followin,g equation

= I.2a ±

Where 5, = the effective width,

a:? thedistance.of the centreof gravitv of theconcentratedload from the thee of the cantileser support,and

= the breadth of concentrationarea of load, i.e., thedirnertsionof the lyre or track contactarea over theroad surface ofthe. slab in a direction parallel tothesupporting edgeot’ the cantilever plus twice thethicknessot’ wearing coat orsurtbce finish abovethestructuralslab.

Providedthat the effective widthof the cantileverslabshall not exceedone—third the length of the cantileverslab measuredparallel to the support.. And pros~’idedfttrther that when the concentratedload is placed nearone of thetwo extreme.endsof the length of cantileverslab in the direction parallel to the support, theeffective width shall not exceedthe above,value, norshall it exceedhalfthe above value plusthedistanceofthe concentratedload from the nearer extreme. endmeasuredin the direction parallel to thefixed edge.

(ii) Fortwo or more concentratedloads

If the effective widthof slab for one load overlapsthe effectivewidth of slab for an adjacentload, resultanteffectivewidth forthe two loads shall be takenas equal to the sum of therespectiveeffective width for eachload minus the width ofoverlap, providedthat the slab so designed is tested forthe twoloads actingseparately.

Note : Slabsdesignedon the above basis need not be checked forshear.

(3) For ribbedslab orthrough slab otherthan solid slabs

54

[FtC:2 1-2000

‘]‘he effective width shall dependon the ratioof the transverseand longitudinal flexural rigidities of the slat,. When this ratiois one, that is when the transverse and longitudinalflexuralrigidities aie approximatelyequal, the valueof effective widthas foundfor solid slal,s may le used.But as the ratio decreases,a proportionately smaller value shallhe taken.

105.l6... Dispersion of loads along the span ‘ Theeffect of contactof wheel or track load! in the directionof spanlength shall b.c taken as equalto the (!inlcnsion of the tyre.contactareaover the wearingsurfaceof the slab in the directionof ti-ic spai’t plus twice theoverall depth ofthe slab inclustve ofthe thickne.ssof the wearing surfitce.

305.16.4. Dispersionof’ loadsthroughtills and wearingcoat The dispersionof loads’ through fills and wearing coatshall beassumedat 45 degreesboth along and perpendiculartothe spa.n.

105 I 7. Stiffening UnsupportedEdgesof Slabs

105.. 1 7 I . I.Jnsupportededgesof all slabsshall be suitablysti tThncd as indicatedin the following Clauses

305. 17.2. Each unsupported edgeof a slab parallel totraffic and beyondthe clear road width, shall be s.o stiffened asto give a resistingmoment for any typeof flexure equalto or inexcessof lhat of a 300 mat strip of the main roadway slabadjoifling the edge.In caseof a roadwayslab of unifonit depth,whether thereinforcementis’ one—way(parallel to or across thetraffic) d)~two—way, the maximum resisting moment of theroadwayslab adjo’ining the edgeand given by a 300 mm stripin any direction shallhe takenas ti-ic criterion for the resistingmoment or the stiffenededge. Whenthe roadway slab is ofvaryingdepth in thedirectionparallelto the edge concerned, thestiffening at any particularpoint along thelength of edgeshall

55

IRC : 21-2000

be adjustedaccordingto the resistingmoment of the 300 mmadjacentstrip at that particularpoint.

Stiffening of edgemayconsistof a reinforcedkerb section,or an edgestiffening beam.

305. 17.3. Unsupportededgealonga line acrossthetrafficot a roadway slab (as at the cantilever end of a solid slabcantileverbridge) shall, for a strip of at least 300 mn-i width inaddition to the. articulation, if any be suitably stiffened byproviding top and bottomreinforcementacross’ the directionoftraffic. The sectionalareaof such reinforcementfor a 300 mmwidth of this strip, both at the top and bottom shall eachbe notlessthan theaverageareaof longitt.idinal reinforcementfor 300mm width at the ct-id of the cantilever.

305.17.4. For cantileve.rshtb, minimum reintbrcementof4 rtos. of 16 mm clia HYSD barsshallbe providedparallel to th.efree edgeat 150 mm spacingat the tip divided equallybetweenth.c top and bottom st.irfaecs.

305.18. Distribution Reinforcement in Slabs

305.18.1.For solid slabs spanning in one direction,distributing bars shall be provided at right anglesto the maintensilebars:’ to provide fbr the lateral distribution of loads. Thedistribution reinforcementshall besuchasto producea resistingmomentin the directionperpendicularto the spanequalto 0.3times the momentdue to concentratedlive, loads pius 0.2 timesthe moment due to other loads such as deadload, shrinkage,temperature,etc.

30S 1.8.2. In cantileverslabs andin portionsof supportedslabs which never experience sagging moment, the totaldistribution steel~ihallbe distributed half at the top and half at

56

IRC : 21-2000

the. bottomof the slab.. For cantileverslabsthe distribution steelshall be calculatedon the basis of 0.3 times the live loadmoment pius 0.2 times th.c dead toadmoment and for portionsof supportedslabson thebasis of momentsindicatedin Clause305.. 18.!.

305. 19. Minimum Reinforcement in Beamsand Slabs

The areaof tensionreinforcementin a beamshall be notIe.ssthan 0.2 per centofbt.d. when usingFe 415/500gradebarsor 0.3 per centof ht.d. whenusing Fe 240 gradewherebt is thebreadthof sectionand d is the effectivedepth.For a box, 1’ orI section ht shall be takenas’ the averagewidth of concretebelow the upperflange.

In slabs’ ti-ic tension reinforcementshall be not less than0. 12’ per cent of the total cross-sectionalarea when. usingFe 4 15/500gradebarsandnot lessthan 0.15 per centof the totalcross-sectionala.reawhen using Fe. 240 gradebars.

30 5 ..i?0. Bending Moments

The bendingmomentsto be providedfor at a cross—sectionof ~i continuous beam orslab should be. the maximum positiveand negativemomentsat such cross—sections,allowing, in botheases’, if’ so desired,‘for the reducedmomentsdue to the widthof the supports,for thefollowing arrangementsofsuperimposed.loadings’

(i) alternatespansloaded and all otherspansunloaded,

(ii) any two adjacent spansloaded andall other spansunloaded,

Except wherethe approximate valuesof bendingmoments’are. used, the negativemomentsat the supports forany as.su’medan’angementof loading may eachbe increasedor decreasedbynot moreth%m iS per cent, providedthat these modifiednegative

57

IRC :21-2000

moments are used for the calculation of the correspondingmomentsin the span.

306. COLUMNS AND COMPRESSIONMEMBERS

306.1. Classification

Columns can be classified under the following threecategories

U) Pedestalcolumns : Ratio of effective length to least radiuso’l’gyration less than 12.

(ii) Short columns ‘Ratio of effective len.gtl to least radius ofgyration more than i 2 but less than 50.

(iii) Long columns : Ratio of e.fk.ctiye length to least radius ot’gyration more than 50 but less than 150.

306.1 .1. For the purposeof calculating ti-ic radius of

gyration ‘for this clause,the cross—sectionof the column forc.:olumns with hinders and irt the caseof columnswith helic211rciniforccrncn.t,the sectionof thecorewithin the outersurfisceofthe helical reintbrc.emcntshall be. considered.

306:. 1 .2. For purposeof this Clausethe effective columnlength given in. Table 1.2 should be used,where 1 is the lengthof the. column, between adequatelyrestrained supportii. Theeffective column length values given i’n this Table 13 arc inrespcc.tof typical casesonly and embodythe generalprincipleswhich sI’tc uldi he employedin assessingtfteappropriatevalueforany particularcolumn.

306.2. Longitudinal Reinforcement

306.2.i. In a pedestalcolumn the cross-sectionalareaofiong~tudinairein’fbrcemcntshall be not less ti-ian 0.15 percentofthegrosscross-sectionalareaof thecolumn andin column other

58

IRC : 21-2000

Table 13. Effective Column Length

Type of column

Properly restrainedat both ends inposition and direction

Effective column length

0.75 1

Properly restrainedat both endsin.

position and imperfectly restrained

in. direction at oneor both ends

Properly restrainedat one endinposition and directionand imperfectlyrestrainedin both position and

direction at the other end

A value intermediate between

0.75 / andI depending uponthe

efficiency of the directional

restraintA value intennediatebetween /and 2/ depending uponthe

efficiency of the imperfectrestraint

Notes: Effective length of piers shallhe as follows

1,1) Only pier condition (without superstructure)— 2 /

Superstructureon one side condition — 1.75 /

Service condition with s nply supportedspans on both sides.and pier tixed at base- 1.2 /

Where/ is the le’ngth of piers betweensurfhces of restraint.Forahutrncntsthecondit~on.s( 1) and (2) will generally apply.

than pedestalcolumns, the cross—sectionalareaof longItudinalrcinlbrc.ementshall neitherbe less th.t.tn 0.8 per cent nor morehan 8 per centof th~egross sectionalareao:f the. column.

.306.2.2. in any column other thanpedestalcolumns: thathas a largercross—sectionalarea than the minimumrtxiui red toresist the loads lncludlng the horizontal threes, theminimum

pcrcen.taeof steel shah he based uponthe area of’ concreterequ~red to resist the direct stressand not upon the actual area.in any case thearc.aof stcc.l providedshall be not less than 0.3

per cent of ti-ic gross sectionalarea.of concrete.

(2.)

:3

59

IRC: 21-2000

306.2.:., A short or long rectangularcolumn shall haveaminimum of four longitudinal bars.. In caseof columns withother than rectangularsections,there shall be onelongitudinalbar placed near each angle pointof the column.

Short and long columns with helical or circular hindersshall haveat leastsix longitudinal bars.

306.3. Transverse Reinforcement

306.3.!.A reinforced concretecolumnshall have transversereinforcement in the form of lateral ties, circular rings or ahelical. They shall be so arrangedthat every corner barandalternatelongitudinal bar near die faceof thecolumn shall havelateral supportprovided by ti-ic corner of a tie having anincludedangleof not niore than135 degrees andno bar shallbefarthe.r than 1 50 mm from sucha laterally supportedbar.. Wherethe bars arclocated aroundthe peripheryof a circle, a. completecircular tie may he used.

306.3.2. The diameterof transverse reinforcementof anytype shall not ordinarily he lessthan one-quarter thediameterofthe largestlongitudinal bar in that region of the colunin and inrio caseless than 8 mm.

306.3.3. The pitch of transverse reinforcement shallnotexceed300 mm or the lesserof the following two dimensions

.0) The least lateral dimensionof the column.

(ii) Twelve times the diameter of the smallest longitudinalreinforcementin the column.

306.3.4. Where.thepitch actually adoptedis less than thepermissihle maximum as per Clause306.3.3,thediameterof thetransversereinforcementmay be reducedfrom that given inClause30(5.3.’2 subjectto a minimumof 8 mm. But the volume

60

IRC: 2 1-2000

of lateral reinforcementcalculatedon the basis of diameterandmaximum pitch given by Clauses 306..3.2 and 306.3.3rt~spectivcly,shall not bereduced.

306.3.5. in caseof helical reinfbrcementintended for theincreasedload capacity’ as per Clause306.5.2, the heiica.1reinforcementshall in addition to Clauses 306.3.2, 306.3.3and306.3.4s.atis.fy the following requirements

fi) l’telica.l reinforcementshall consistof evenly spaced helicaisand shall have their endsanchored.The splicing of the helicalsshaHbe madeby welding or by a lap of one anda half turns.

(ii) The pitch of the helicats shall notbe more than 75 mm normore than onesixth the diameterof the coreof the column.

306.4. PermissibleI..~oadon Axially Loaded Coiumns

306.4,1. On a ~shortcolumn, reinforced with longitudinalbarsand lateral ties, thepermissibleaxial loadN on the columnshall not exceedthe valueobtainedfrom the equation

~ +~‘j~4C C &C C

Where cro=: the permissiblestressin direct compression for concreteas given in Clause303,

the permissible stress in direct compressionfor thelongitudinal steel as given in Clause303.2,

= the cross-sectionalareaof concreteexclusive of anyfinishing material applied after the casting of thecolumn and exclusiveofthe areasof longitudinal steel,and

A, = the cross-sectionalareaof the longitudinal steel.

306.4.2. On a short column reinforced with longitudinalhl.trs andhelicals complyingwith Clause306.3.5,the perrñissiblc~:ixi~tiload N on the column shall not exceedthat given, by theequation in Clause 306.4. 1 or by the equation given below

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IRC 21-2000

whicheveris greater

N = a oil o ±a 44 -f 2 a .AC C SC .5 ~r 95

Where ito rr the cros:s-scctionalarea of concrete inthe column core,excluding the areaof longitudinal steel,

= the equivalent area ofhelical reinforcement(i.e., thevolume of helical reinforcement perunit length of thecolumn), and

a = Tensionin helical reinforcement

The sum of the terms coA o and 2 a A , shall notexceed 0 S fkAc where i~ the characten4ic ‘sflcngth ofconcrete.

306~43.In case of a long column reinforced withlongitudinalbars’ and tiesor helicalreinforcement,the pemiissibleaxial load on the column shall be obtainedfrom the equationsgiven in Clauses 306,4.1 and 306.42 respectivelyprovidedreducedvaluesof permissiblestressfor steel and f’br concreteare taken uch reduced valuesof the maximum permissiblestressesshall he. obtamed by multiplying the appropriatcmaximum penmsstbiestressesgiven in Clauses 306.4. I and306.4~2by the co—efficient B given by the equation

[OUr

wl~ie.re C: the reductionco—efficient,

‘1 ..:;;.; the’efiè’c:iive length ot’ the column, and

c. the least radiusof gyration

Note When in a column having helical reinfbrcement,the perrnis:sibleload is basedon the core area, the radius c: I gyration shall alsobebasedon the di arneterof the cure,

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IRC: 2 1-2000

306.5. Design of Sectionsfor Combined Axial Loadand Bending

306.5.i. When reinforced concretesection underaxialcompressionis subjectedto bending in one or more directions,

the sectionshall be designedby any recognised rationalniethodor by the method given in Clauses306..5.2 to..306.5,4.

306.5.2. The maximum direct stressand bendingstre’ssin t.he section shall be calculatedby the following methods

(1) Direct stTess

fi) For columns with helical reinforcement complyingwith Clause306.3.5.

NDirect stress

,4o ‘5 ccAs 2.cx.14sp

(ii) For columns withtransverse,reinforcement otherth.artthat in (i) above.

NDirect stres’s

A ‘f’ cx. A

.4,!

(1) Bending stress

N the k:nu.l on the column in the directionof its axi.s’,

‘1 the area ol concretesection perpendicularto the .L9:M isof the c.cfluniri

the modular ratio,

the. scct~oumodulus (in case o t’’he’nding in

d~reetinns,,W is tlie’ sectionniodt.iiu.s w~threftirence to

the appropriate principalaxis for two—wa bending),and

:4’.! Dl0111011

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1RC 2 1-2000

in caseof rectangular sectionsubjected tobending in two directions, the expression

ffv in theequation(2) abovecanbe substitutedby

IV!ii

Where .4’!~and A! are the betding moments abouttwo principalaxis of the section and B’, and W iu~ethe correspondingsectionmoduli.

306.5.3. If the directand the. bending stresses, calculatedfrom cquatiotisgiven in (1) and(2’) ol Clause306.5.2 satisfyallthe. following conditions, the sectton may beconstdcnedsafe

ca,cal C,, cal

0) +CC

Where c~o,cal the calculateddirect compressivestress,

GO C:; the permissible direct compn”ssive stress,according to Clause 30.3,’ multiplied by thereduction factor givenin Ctause 306.4.3.

ci,, cal — the. calculated ilexural compressive stress,and

the pennissible tiexural compressivestress,according to Clause 303, muitipl~edby thereduction ‘co-efficient given in Clause 306.’4.3.

(ii) The resultant tension due to direct compressionandflexure is not greate.rthan the valuespecifiocl in Table.11 for the appropriategrade of concretemix,

306.5.4. If the conditiongiven inClause306.5.3 (ii) is riotsatisfic.d, the sectionshall be deemed to havecracked in the.

region of tension and the tensile resistanceof concrete is

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IRC 21-2000

ignored altogether. The maximum stressesin concrete and steelshall the.n be found according to the recognised theoryofcracked section.The fibre compressive stressin concreteshallnot exceedthe valuesgiven fbr flexural compressivestressasgiven in Table9 and thosefbr tension in steel shall not exceedthe pennissible.stressin reinforcementgivert in Table i 0.

307. FOOTING OR BASES

307. 1. Allowable Presstire on the i..4oaded Area

The allowable bearing pressureaCC with near uniformdistribution on the loaded area of a footing or base. under abearingor coiun’m shall be given by the followingequation

C,,. = ~~,‘‘: ‘~ j”

Where no = thepermissibledirectcompressivestressin concreteatthe bearing areaof the base.

C; dispersedconcentricareawhich is geometrically similarto the loadedareaA, and also the largest areathat canbe containedin the planeof A, (maximumwidth ofdispersionbeyondthe loadedarea faceshall helimitedto twice the height)

= loaded areaand,

The projectionof the baseor footing beyond the’’lace ofthe bearing orcolumn supportedon it shall not be less’ than 150mm in any direction.

Adequatereinforcementfor spalling and bursting tensionshall he provided with small diameterreinforcementat closetpacing.

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IRC 21-2000

307.2. Desigit of Footing

307.2,1. Bending moment in footing : The bendingmoments at any sectionofbasefor a reinforcedconcre.tecolumnor wall shall be taken to be the momentof the forces over theentire areaon one side of the section.The critical section forbendingin the basesshall be takenat the faceof the column or

ss’al1.

307.2.2. Shear

30~7.2.2.1. The shear strength of the footings - shall bego.vernedby the severerof the following two conditions.

(a) The footing actingessentiallyas a wide beam,with a criticalsectionextendingin a plane acrossthe entirewidth; the criticalsection for this conditionshall he assumedas vertical sectioinlocated from the face of the column, pedestal orwall at adistance equal to the effectivedepth of the footing.

(h) Two way action for slab or footing with a critical. sectionperpendicularto planeof slab and so locatedthat its: perimeteris mini mum but need not approach closerthan half th.’e effectivedepth to the perimeterof concentratedload or reac~ionare.a.

307.2.3. ‘Bond ‘The. critical section for checking the

loc..al bond stress’ shall he takento be the same.section.a.s given

~n Claw:’e 30’7.2. 1 andalso all other verticalplaneswhereabrupt.changc.:sin section o~cc.ur.

:107.2.4 Tensile. reinforcement The reinforcement

provided to resist the bendtngmomentsdeterminedin Clause307,2, 1 shall be. distributed uniformlyacrossthe full width ofthe section except that in rectangularfooting.s for columns orp~edcs’l’ais,the rcinfbrccment parallel to the short edge shallhemore’ closely spaced near the column or pedestal in a band

width, equal to the short side of the [doting. Thereinforcementirt band width shall be asgiven below

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JRC: 2 1-2000

Reinforcement inband width

Total reinforcementin short direction (13 H

Where 13 is’ the ratioof the’ long sideto theshort side~of’thcfboting.

:307.2.5. Designof pile caps

307.2.5.1. General The pile caps shall he designedeither by tnr..:’s analogy orby bending theory.

307.2.5.2.Design by’ trussanalogy The thicknessof pilecap shall he so ‘propot-tioned to act as sti if member. Theminimum thiclc.nessof capshall he 0.5 times.’ the spacingo’i the

Ic where thereare two rows of piles.

The truss’ should he a triangular form with a nodt:~at thecentreof loaded area. The lower node of the truss lie at the’intersection of the centre line, of the pile.s with the tensionreinforcement. When the truss method issued with widelyspacedpiles (spacingexceedingthreetimes the pile diameter)only the reinforcementwith in 1.. 5 times the. pile diameterfromthe centre ofa pile shall heconsideredto constitutea tensionnurnher..

Eighty pc~r cent of the total reinforcement shall hec.oncentrale,din s.trip...s linking the pile h.ead.s and the remainderuniformly distributed throughoutthe. pile cap.

No checkibm shearis requiredto becarriedout in casepilecapsare designedby trussanalogy.

307.2.5.3. Design by bending theory The bendingmoment at any section of the cap for a reinforced concre.tecolumn or wall shall be taken to be the momentof t’he forcesover the. entire areaon one side .of the section. Thecritica]

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IRC :21-2000

sectionfor bendingin the capshall be takenat the faceof thecolumn or ~vaIi

3O7.2.5~4.Check for shear The critical section forcheckingshcar shall be takenat a distanceof effective depth(‘corn the face of the column or wall for caps acting in one

d~rccttonand at a dtstanceof half the effective depth for capsacting in two directions,

Shearon any sectionfor a pile cap shall be in accordancewith the following

(a) Entire reaction from any’ pile where its centreis located at adistancehalfthe pile diameteror moreoutsidethe section shallhe consideredas producingshearon that section,

(is) Reactionfrom any pile wherecentreis locatedat a distancehalfthepile diameteror more inside the sectionshall he consideredas producingno shearott that section.

(c) For intermediatepositionsof pile centre,the portion of the pilereaction to he consideredas producing shearon the sectionshall he hasedon straight line inter-polationbetweenfull valuefor condition (a) and zero value for condition(b).

1O7~2~5.5.Check for punching shear The punchingshearshall hc~checkedboth aroundthe column and the pile ona perimeter,as shown in Fig. 12.

i)i9n~etc~rof Theror PUc‘Thkkiessof (‘up

h “ Brcuctth of Nor

Fig. 12. Locations of sectionsfor checking punching shear

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IRC: 2 1-2000

The. punching shearshall not exceed0.16 j71

308, BALANCED CANTILEVER AND CONTINUOUSCONSTRUCTION

308. 1. Moments

308. 1 . 1. At sections‘where the dead. load momenti~’ofasig,n opposite to that of the live-load moment, thed.esignbendingr.’ioment tom this condition of loading shall be takena.sequal to the live load momentplus. C). 7 deadloa.d moment,withproper signs. This condition of loads shall he allowed ‘for indesigni.n addition to the othernonnalload conditions.Howeverat least 2.0 per centof the maximumnegative,reinforcement atthe support shall he carriedout right across thespan evenif notrequiredl.y design calculations.

308.2. ArticuLations

:308.2. I , The design of hearings atarticulations: shallensurethat concentratededge stresseswill not he inducedandthe, angularrotation of the cantileversand the suspendedspanis

Possible iv ithout any’ damageto the articulation.

308.2.2.. The slopeand arrangementof reinforcemnentofarticulation shall he as in the’. Fig. 13(a) arid (b).

30K3. Distribution of Reinforcementin the Ten!.”ileFlange

Where the. flanges of T-he.ams aresubjectedto tensilestme;sses in longitudinal direction, the barsof the principalmeii,iforce,ment shall he suitably distributed along the flangeswith somne concentrationat the positionof the web.

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IRC : 21-2000

309. BOX GIRDERS

_I_,__ Lfl*M~In

30~.1. Effective CompressionFlangeWidth

For calculatingstressesat any sectionof a box. girder theeffective:width oftheslabilinctioning asthecompressionflangeshall be in accordancewith Clause305.15.2.

309.2. SoRB Slab Thickness

The minimum thickness of the bottom flange shall bedetenninedby the maximum allowable stressesspecified inClauses303.1 and 303.2but in no caseshall it be less than 200mmor one-twentiethof theclearspanbetweenthemain girders,whichever is greater.The thicknessof the deck slab and websshall not he less than that stipulated in Clause 3052.

‘At r

c~i(VAih Ia U

Fig. l3~h)~.Articulation (generalshapeand arrangement)

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IRC: 21-2000309,3.Reinforcement of Soffit Slab

Minimum reinforcement of 0.5 per cent of the flangesection for Fe 240 gradebar or 0.30 per cent of the flangesection forFe 500/415 gradebarsshall be placedin transversedirection distributed equally over bothsurfaces.Bar spacingshall not exceed300 rnn~.These bars shall be bentup into theexterior girder stemsat least tentimes the bar diameter.In thelongitudinal direction, thelongitudinal steel neededfrom stressconsideration maybe distributed equally betweenthe top andthebottomsurfacesand also it must be ensuredthat eachsurfacehas atleast the minimum reinforcement specified in Clause305.10.

309.4. Reinforcement in the top flange in a directiontransverse to the girders shall extend to the exterior face of alloutside girders, and a minimum of one-third of such reinforcements.hall either be anchored with 90°bends or extendedbeyond thegirder face a sufficient distanceto develop the strengthof thebar in bond provided the flange projectsbeyondthe girders facea sufficient distance to providethis bond length.

309.5. The minimum clear height inside the boxgirdersshall be 1.5 m to facilitate inspection.

310. WORKMANS1.IP

310.1. Concrete

Measurement of ingredients: All the ingredients of concrete,i.e., cement, fine and coarse aggregates and water and admixtures,if any, shall be determined by weight. Due allowance shall bemade for the weight of moisture present in the aggregates.

310.2. Proportioning Ingredients of Concrete

3.10.2.1. The proportions of cement, aggregates ant

71

IRCH 2 1-2000

waterniay he determinedby any of the recognisedmethodsofproportioning provided that all the requirementsof Clause303are satisfied.

310 2. Accuratecontrol shall be kept on the quantityof mixing water, which when specified, shall not be changedwithout the approvalof the competentauthority.

310,3. Mixing Concrete

All concrete.shall be niachine-mixe~In order to ensureuniformity andgoodquality ofconerete,the ingredientsshall bemixed in a powerdriven batchmixer with hopperand suitableweigh batchingarrangenientor in a central mix plant.

The concorteshall he mixed until it is of evencolour anduniform consistencythroughout.In no caseshall the mixing bedone ‘for lessthan two minutesafterall the ingredientsarein themixer.

The addition of water after the completion of the initialmixing operationshall not be permitted.

310.4. Transporting Concrete

3lft 4.1. Mixed concreteshall be transportedfrom the

~~~accc,~fmixing to the place. of final deposit as rapidly aspracticableby methodswhich will prevent the segregationorloss of the in~~redientsanddepcsited asnearaspracticableto itsfinal position to .avoid rehandlingor flowing.

Concretemaybe transpori.edby transit mixersor properlydesignedbucketsorby pumping.Transitmixersorotherhaulingequipment when ‘used shoi.ild be equipped with in cans ofdischargeo’f concretewithout segregation.

The maximumperiod betweenthe addition of waterto the

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IRC 2 1-2000

mix and theplacing and compactionof concrete in finalpositionshall not exceed30 minutes or the initial setting time of thecementwhicheveris less, exceptingin the casewhere retardersare used.

3 10.4.2. When concrete.is conveyed by chute, the plantshall he of such size and design as to ensure practicallycontinuous flow in the chute. The slope of the chuteshall hesuch. as to aIlow the concreteto flow without the useof ancxc.essive quantity of water and without segregationof theingredients.The delivery’ end of the chute shall be as close aspossibleto the point of deposit.The chuteshall be thoroughly‘flushed with water beforeand after each workingperiodand thewater used forthis purpose discharged outside the foniiwork.

310.5. Placingand Compacting Concrete

310.5.1. The concreteshall be placedbefore settinghascommencedand shall not he subsequently disturbed unlessretcmperingis speciallyperniitted. Concreteshallbe.~soplaced.as to avo.id segregationof the materialsand displacementofreinforcement.To achieve this. concrete should be lo\veredvertically in the fonns and horizontal movementof concreteinside the forms should as far as practicable be brought to aminimum. In wall forms dropchutes attachedto hoppers at thetop should prekrablyhe usedto lower concreteto thebottom ofthe form. l...Jnde.r no circumstancesconcrete shall be droppedfreely from a heightof more than2 metre.

3 10.5.2. Concreteshall be thoroughlycompactedduringthe operation of placing, and carefully worked around thereinforct~ment,aroundembeddedfixtures andinto the cornersofthe ‘form work,. To achievepro’per compaction vibrators shall heused. The vibrator c.an he internal or’ external types anddependingupon theshape.and sizeofthe memberboth th.e types

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LRC: 21-2000

together shall be used. When internal vibrators are used theyshall be inserted vertically to the flu depth of the layer beingplaced and ordinarily shall penetrate the layer below for a fewcentimetres. The vibrator should be kept in place until airbubbles cease escaping from the surface and then withdrawnslowly to ensure that no hole is left in the concrete, care beingtaken to see that it remains in continued operationwhile beingwithdrawn.

The internal vibrators shall be inserted in an orderly

manner and the distance between insertions should be about i~

times the radiusof the area visibly affected by vibration.

310.5.3. Construction Joints

310.5.3.1. Concreting shall be carried out continuouslyupto constn.ictionjoints, the positionand arrangementof whichshall be pre-determinedby the designer.

31 0.5.3.2. For a vertical construction joint, a stoppinghoard shall b.c fixed previously at the pre-determined positionand it shall be properly stayedto havesufficient lateral rigidityto preventdisplacement or bulging when concreteis compactedagainstit. Concretingshall be continuedright up to the board.The hoard shall not be removed before the expiry of the periodspecified in Clause 310.10 for removal of vertical forms.Preparationof the concrete surfaceandresumptionof concretingshall he in accordance with Clause310.5.4.

310.5.4. Before resuming work at a construction jointwhenconcretehas not yet filly hardened,all laitance shall beremoved thoroughly,care being takento avoid dislodgementofcoarseaggregates.

lYhen work has to be resumedon a surface which has

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IRC : 2 1-2000

hardened, it should be thoroughly hacked, swept clean, wettedandcoveredwith a layerof neat cementgrout. Theneatcementgrout shall be applied to the surfaceand this shall he followedby a 10 mnni thick layerof cementmixed in the same proportionasthat of cementarid sandin concreteand the concretings.hallbe resumedimmediately thereafter.The first hatchof concreteshall be rammedagainst theold work to avoidformation o.f anystone pockets, particularattention being paid tocorners andclosedspots.

310 5. Thesequence.of concreting, strikingof formsandpositioning of bearingsfor every individual structureshall hedecided,well in advanceof the commencementof work.

310.6. Spacersmay be embeddedin concreteprovidedthat the qualityofthe effectivecoverof any part of the structureis not reduced belowthe standardrequired by this code. Toensure adequatecover use of manufactured chairs: isrecommended.

310.7. Ctiring and Protection of Concrete

Theconcreteshouldhe kept constantlywet for a minimumperiodof 14 (fourteen)days exceptfor rapid hardeningcementconcretewhen it can be reduced to5 (five) days.Water shouldhe applied on un’fornie.d surfhcesas soon as it can bedonewithout marring thesurfaceand on formed surfiices immediatelyafter the forms: are stripped. The concrete. ~ I he keptconstantlywet by’ pond~ngor coveredwith a luer of sackin~g.,canvas’, hessian or a similar absorbentmaterial and keptconstantlywet.

After placing and during the. first stagesof hardeningconcrete shall he protected from harmful effects of’ sunras,drying winds, cold, running water, s:hocks, vibrations, trafficincluding constructiontraffic etc.

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1RC 21-2000

310.8. Concreting in Extreme Weather andConcreting under Water

310.8.1. Concreting in hot weather : If concreting hasto be done in hot weatherat atmospherictemperature exceeding40°C,precautionsshall be takento ensurethat the ingredientsofthe concreteare maintairedbelow 40°Cand curing is startedhelore the concretesurface dries out and is satis:fiictorilycontinuedthroughout the requisite period.

3,10.8.2. Concreting in cold weather

31(18.2.1. When depositing concreteat or nearfreezingtemperature, precautionsshall he taken to en~~tirethat theconcreteshall havea temperature.of at least5°Cwhile placingand that temperatureof the concreteshall be maintainedabove4°Cuntil it has thoroughly hardened.

310.8.2.2. Whennecessary,concreteingredientsshall hehe,atedbefore niixing but cenientshall not he heatedartificiallyo.therthanby’ the heattransmittedto it from otheringredientsofthe concrete.In general, heatingof the mixing water only toabout 66°Cmaysuffice for this purpose.

310.8.2.3.No frozen materialor materials containingiceshall be used.

31(18.2.4.All concrete damagedby frost shall he removed,

310.9. Steel Reinforcement

310.9.1. Bending : Reinforcementshr.ll not he. bent orstraightenedin a manner that will injure the material. Allreinforcenientshall be bentcold.

310.9.2. Placing : Al I reinforcementshall he placedandmaintainedin positions shown on the drawings. -

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IRC: 2 1-2000

310,9,3. Splicing and lapping Splicing and lappingshall he in positions conforming to the drawingsamid for this

purposeall reinforcing barsshall be to the full lengths indicatedthereon.The competentauthority may, however,makesuitableadjustmentin the locationsof the splices to accommodate theavailable lengthsof bars. En all casesof such adjustment, therequirementsof lap length and other stipulations, as per Clause304.6.6 must be compliedwith.

310.9,4. Substitution of bar sizes : in other toaccommodatethe availablesize of bars, useof bar size otherthan those shown on the drawings maybe permitted by’ thecompetent authority. in case of such substitution, thereinforcementactuallyused shallhavean areaequivalentto theoriginal or slightly in excess thereof,provided furtherthat thevarious stipulations of this Code are not violatedby suchsubstitution. The requirementswith regard to bond stress,limitationsof bar sL~..es,spacingof bars, and cover, etc.,shall he

specially looked into.

310.10. Falsework

For design, fhbrication, erection and strippingof falsework, IRC:87 “Guidelines for the Designand Erectionof FalseWork for Road Bridges” shall be followed,

The form work should he robust and strong and the jointsshould be leak proof. The staging,scaffoldingand shutteringarerequiredto be properly designedso that theirerectionas well asstriking can he conveniently done. The design shoulda~oensurethat at the time of striking, the concrete doesnot getdisturbed and the forms are conveniently removed.Use ofbamboostagingshall be prohibited.

‘Ballies’ staging shall he checked for its soundnessand

77

LRC :21-2000

shall be properly braced to make it rigid and strong. Verticalsplicing of ballies shall not be permitted.

Wherecenteringtrussesor launching trussesare adopted‘for casting of superstructure,the joints of the centering orlaunchingarrangement,whetherwelded,revertedor bolted shallbe thoroughly checked and various membersof the centeringtrussesshall be examinedfor properalignment and unintendeddeformationbefore proceedingwith the concreting.

The location where fixing of reinforcement and placing ofconcreteart~.being done, shall be accessibleto the inspectingofficers at all stagesof construction.

310.11. Tests

310.Ii.!. Tests of cement, aggregates,water etc., tosatisfy the requirementsof Clause302 shall be carriedout fordesigning the mix of design mix concrete in accordance withrelevantISI Codes..Thesetestsmay also be carriedout whilearriving at theproportionsof ingredientsin thecaseof nominalmix concreteif required by the competent authority.

310.1i .2. Testsof reinforcing steel,cement, aggregatesand wateractuallyusedat site for production of concrete plainor reinforced and if any gradeshall becarriedout to satis&therequirementsof Clause302 and in accordancewith relevantIS!Codes.

3 10.11.3. Load tests on the completedstructuremay becar,iedout if specially desired.

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IRC: 21-2000

Appendix /(303~4~2)

DESIGN SURFACE CRACK WIDTH

AA. The design crack width may be calculated from thefollowing equation

3acr ~‘m

Design surface crack width = 1+ 2(2~. Crn~r,)

(h—x)

Where c~ is the minimum cover to the tension steel;

= is the perpendicular distance from the pointconsideredto thesurfaceof thenearest longitudinalbar;

= is the averagestrain at the level where cracking is

being considered calculated allowing for thestiffening effect of the concrete inthe tensionzone; this may be obtained from the equation

[12kh(a’—x)]l0~~“ A,i~h—x)f,~

Where b1 = is the width of the section at the centroid of the

tension steel;

= is the distance from the compression faceto thepointatwhich the crackwidth is beingcalculated;

h = is the overall depth of the member;

= is the averagestrain at the level where cracking isbeingconsidered,calculated ignoring the stiffeningeffect of the concrete in the tension zone;

x = is the depth of the neutral axis found from theanalysisto determine e~

A = is the areaof tensionreinforcement.

79

mc 21-2000

A negative, value of ç indicates that the section isuncracked.In assessingthe stra.ins the modulusof elasticity ofthe concreteshould be taken as half the instantaneousvalue.

A.2. Permissible crack width: The crack width undersustainedloadsshall not exceed0.2 mm for severeconditionsofexposureand 0.3 mm for moderatecondition o’f exposure.Theconditions of exposureare gi ~‘enin Table 4 and the sustainedloadsniay be takenasdead loadpius 50 per centof live load.

80